Window head height changes with sill?

[Anonymous]

Hi all,
Running in to a small but annoying issue with my windows in AC20.
Both windows have all the same settings for head height, 20mm reveal, anchor setup etc, the only difference is that one has a brick sill and the other does not.
For some reason, the one without the sill is lifted up 20mm (same as reveal size) and ends up with the incorrect head height when measured.
Is this normal? Is there any way to correct this without fudging the head height value?