Normally, when you place 1 ingot of osmium and 1 ref.obs/glowstone dust into osmium compressor it uses exactly 1 ingot per dust. You can then continue place ingots and dust 1 by 1 and all will be OK. But if more than 1 ingot and 1 dust is placed each dust uses 201 liquid osmium not 200 thus losing 1 ingot worth of osmium. This issue is present in versions from 7.1.2 to 9.0.2 and maybe some others.

This is just a thing in the long list of stupid stuff with Mekanism.

But then why is there a loss when putting in more than one, but not when putting in single ones?
Makes not a lot of sense imo.

This loss of osmium efficiency has been intended as far as I'm concerned.