Why every page says the same thing
Almost all birthday pages are written from a table of date ranges. A table like that contains one piece of information, the sign, and the same entry covers thirty consecutive dates. Anything written from it will therefore read identically for 27 March and for 5 April.
What a table cannot give is a position. On this date the noon Sun has stood between about 5.9 and 7.3 degrees of Aries in every year from 1930 to 2035, which is a fact about the date rather than about the month it sits in.
The decan is missing for the same reason. It is a ten degree division rather than a thirty degree one, so it cannot be read off a date range at all, and a page working from one has no way to mention it even if the writer wanted to.
How the position stays fixed
The tropical year is about 365.2422 days long and a common calendar year is 365, so at the same hour each year the Sun has moved about a quarter of a degree further on. Every fourth February adds a day and hands almost all of that back.
The consequence is that a birthday's solar degree oscillates inside a band roughly a degree and a half wide instead of wandering off over a century. Your own year fixes where inside the band you fall, and the widget above computes it exactly, together with the distance from your Sun to the nearest sign boundary.
The decan under it
Below ten degrees of Aries is the sign's first decan, which the Chaldean order assigns to Mars. Since Mars also rules Aries, the sign ruler and the band ruler are the same planet here, which is unusual and which the tradition reads as the sign at full strength.
That reading belongs to the tradition and should be labelled as such. The band is a measurable ten degrees of longitude, the same in any ephemeris you consult. What is said about its character comes from inherited texts, cannot be checked, and does not become more reliable by being repeated on a birthday page.
What the band does supply is a real narrowing. A sign covers about thirty days of births and a decan about ten, so knowing the band separates people born at the start of Aries from those born at its close. That is a smaller claim than most birthday pages make and a more defensible one.
Where the individual detail lives
The Moon changes sign every two and a half days or so, so on this date it might be in either of two signs and occasionally a third. Only an hour of birth resolves it.
The Ascendant needs the same hour plus a birthplace, since all twelve signs rise in a day and the timing depends on latitude. Two people born on the same 27 March in the same hospital, six hours apart, will usually have different rising signs and different house layouts.
Stated plainly, the sun sign is the least personal part of a chart. It is worked out from the coarsest piece of information available and shared with roughly a twelfth of the population, while the placements that actually differ between people all need a time.
Questions people ask about this
Is 27 March always Aries?
Yes, in every year from 1930 to 2035 at noon. The nearest boundary is six or seven degrees behind, which is about a week of solar motion, so nothing about the hour or the time zone reaches it.
What degree is a 27 March Sun?
Between about six and seven degrees of Aries. The exact figure depends on your year within the leap cycle and on your birth hour and place, all of which the widget above accounts for.
What is the decan and who rules it?
The first decan of Aries, zero to ten degrees, ruled by Mars in the Chaldean allocation. Some modern sources use a different scheme for the same band, so check which one a page is quoting.
Do people born on the same date have similar charts?
Only in the Sun, and in a rough sense in Mercury and Venus. The Moon, the Ascendant and the houses vary within a single day, which is where most differences sit.
Is the sun sign worth anything at all then?
It is one real placement out of many, computed correctly and shared widely. The mistake is treating it as the whole chart rather than as the piece that needs the least information.
How do I get the rest?
Supply a birth time and a birthplace. Without a time, some placements can be narrowed rather than fixed, and it is better to say so than to guess.