"Summary: For ⌈ and ⌊ GNU APL follows the ISO
standard while IBM APL2 does not."
While this may be the case, I would argue that the APL2 approach is much more numerically robust than the ISO approach.
Within
the range of contiguous integers exactly representable as 64 bit
floats, only 2049 of them will have a different value after having
⎕CT=1E¯13 added to them. For the rest of them, an additional 13 decimal
digits of mantissa bits won't fit into the float, and fudging by an
absolute ⎕CT will be a no-op.
It is hard to argue that floor and
ceiling are "tolerant" if they only do a fuzzy comparison on a few
numbers, and a regular comparison on all the rest.
In this case, it would seem that doing it the APL2 way, and not the ISO way, would be the prudent choice.