bearisgray

these drawings might help explain what happened -

(Sorry about the poor quality - I have a new scanner and am still learning how to operate it)

Basically, what happens:

When one cuts a square - one cuts it 1/2 inch larger in both directions that the finished size to allow for seam allowances.

When one cuts a square diagonally to make half-square triangles - the points add an additional 3/8 inch to the 1/2 inch seam allowance of the original square - so that is why one adds 7/8 inch (one inch usually works better - and just trim a smidge from the sewn unit) to a square that will be cut diagonally to make HSTs.