Why colleges recalculate
Because American high schools weight courses in genuinely different ways. Some add a full point for AP and nothing for Honors; some cap the weighted GPA at 5.0 and some do not cap it; some weight only core subjects; a few run a 100-point scale and convert at the end.
A weighted GPA is therefore not comparable between two schools, which makes it useless for the one thing admissions needs it for. So many universities strip the weighting off, apply their own, or use core academic subjects only — and then compare like with like.
The practical consequence: chasing a weighted number by loading electives that carry weight is optimising for a figure that frequently gets discarded.