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because they're the same length?

.......therefore "ridiculous"

The rate of the bell flare also has something to do with the pitch, although not TOO radically, but that's why brass instruments aren't the same length as organ pipes.

There is also the series of partials to deal with, or why we can play more than one note for any given valve combination. For example, on a BBb tuba, the third partial F at the bottom of the staff and the 4th partial Bb second line in bass clef are both played open. On an Eb tuba, the 3rd partial Bb is the same note pitchwise, and is also played open, but only because it is at a different point in the harmonic series for that instrument. Likewise, on an F tuba, the 2nd partial F is the same note also played open, for the same reason: different place in the harmonic series for that tuba.

As an aside, generally bell flare does not make that much difference in pitch, but when Besson Sovereign went from the small flare rather evenly tapered throat 17 inch bell to the really wide flared 19 inch bell, it caused the pitch to drop so much that the first ones had to have two inches taken out of the bugle to bring it back to pitch.