You're confusing mash efficiency and brewhouse efficiency. This is actually the correct way for efficiency to be calculated when the Brewhouse Efficiency is the determining factor. What you're describing is based on mash efficiency being constant.

Brewhouse Efficiency is the percentage of sugar that makes it to the fermenter. This means that if you have 1 gallon of trub loss, and 4 gallons make it to the fermenter, you have 80% of the sugars extracted getting to the fermenter. Thus, 80% Brewhouse Efficiency is the highest possible number.

If you know you'll get 70% of the total available sugar from the grain, this reduces your brewhouse efficiency from the maximum illustrated above. If you measure your wort at the higher gravity with more volume, then the ONLY possible place to gain that sugar is with better mash efficiency.

If you're more comfortable making your batch size measurements in the kettle, then set trub loss to zero to make the Brewhouse Efficiency the same as Mash Efficiency. A future update *may* have the option to let mash efficiency be the determining calculation. With that, the brewhouse efficiency and OG will decrease with increasing loss to trub.