How shifting(borrowing) works for the following ACP failure?

The ADP test passes if the NHCE ADP is 4.06% (using +2).  That leaves 0.45% (4.51% - 4.06%) available for use for the NHCE ACP % in the ACP test.

The NHCE ACP test passes if the NHCE ACP borrows 0.07% to add to the 1.21% and get to 1.28%.  The ACP test passes since 2 times 1.28% is 2.56%.

Technically, percentages aren't shifted, amounts are. 

This really only matters practically when the populations in the ADP and ACP tests are different, like if the match requires last day and/or 1000 hours.