There is very cool history as to why. In armv5 the BL was two separate instructions. You could take an interrupt in between. And it was documented what each did. Veeery cool. Then they wanted more range. And now that they redefined the two halves as one instruction that could not be in halves, they also redefined a few bits that were always 1 before, to allow the second half of the instruction to be recognized right without a “previous instruction was first half of BL” flag somewhere that would need to be context switched and all that.