Where the position actually is
At noon across 1930 to 2035 the Sun on 11 April has fallen between roughly twenty and three quarters and twenty two degrees of Aries. Every year in that sample is past the line at twenty degrees, so the band is settled by the date alone even though the ruler is not.
The window is narrow for the usual reason. Three ordinary years push the position forward by about three quarters of a degree in total and the following leap day pulls it nearly all the way back, so the calendar and the orbit stay in step across a century.
A birth hour moves the figure by up to half a degree either side of noon, which is enough to change the last digit and not enough to change the band.
The Chaldean answer: Venus
This scheme lists the seven visible planets in order of apparent speed, from Saturn as the slowest through Jupiter, Mars, the Sun, Venus and Mercury to the Moon. That sequence is repeated continuously across the thirty six bands of the zodiac, beginning at the first degree of Aries.
Counting through, Aries takes Mars, the Sun and then Venus. The logic is arithmetic rather than symbolic, which is part of its appeal: once the starting point is fixed, every band in the zodiac follows without a further decision.
The triplicity answer: Jupiter
The other scheme starts from the element. A sign's first band goes to its own ruler, and the second and third to the rulers of the other signs of the same element, taken in zodiacal order. The fire signs are Aries, Leo and Sagittarius, ruled by Mars, the Sun and Jupiter, so the bands of Aries run Mars, Sun, Jupiter.
Note that the schemes agree on the first two bands of Aries and part company only on the third. That coincidence is why the disagreement often goes unnoticed until someone with a late Aries birthday checks two sources.
Neither scheme was invented to contradict the other. They grew from different starting principles, one from planetary speed and one from the elements, and were both preserved because each is tidy on its own terms.
What to do with the contradiction
The reasonable conclusion is that the ruler of this band is a convention and not a finding. Both systems are internally consistent, both are old, and nothing observable arbitrates between them. A description built on either should be read as inherited interpretation.
The measured facts are untouched by the argument. The Sun's longitude on a given birth is a checkable number, and so is the distance from it to the nearest sign boundary. The widget above returns both from a year, an hour and a birthplace, along with the Moon and rising signs that this date leaves entirely open.
Aries remains cardinal fire under Mars whichever band ruler is preferred, so the dispute narrows a description rather than unsettling anything larger.
Questions people ask about this
Is the 11 April decan ruled by Venus or Jupiter?
Venus under the Chaldean order and Jupiter under the triplicity scheme. Both are widely printed, and the disagreement is structural rather than a mistake in one of them.
Which scheme should I use?
Neither has an evidential claim over the other. Most modern popular writing follows the Chaldean order, so Venus is the answer you will meet more often, but a source using triplicities is not in error.
Which decan is this date in?
The third decan of Aries, from twenty to thirty degrees. Every year sampled at noon between 1930 and 2035 falls past the line, so the band itself is not in doubt.
What degree is the Sun on 11 April?
About twenty one degrees of Aries, within a window of roughly a degree and a quarter across the last century. Your birth year fixes where in the window you sit.
Do the schemes disagree anywhere else?
Often. They agree on the first band of every sign by construction and diverge across many of the rest, which is why decan tables from different books rarely match all the way through.
Is 11 April ever Taurus?
No. Taurus begins around the twentieth of April, so this date sits more than a week inside Aries in every year of the modern calendar.