There's two common ways of mid-point rounding: "away from zero" and "to even" (also called "banker's rounding").
Mid-point rounding "away from zero" always rounds up, so each time a transaction in a common direction is rounded, the same party always gains from the rounding up and the same other party always loses out.
Like in your case, if Inno always rounded "away from zero" for the 37.5 goods, you'd always be getting 38 and Inno would always be 'losing out' on 0.5 goods.
Mid-point rounding "to even" gives a 50/50 chance for who gains and who loses each time.
So, in your case if you had 95 goods already and the PP was due to give you 37.5:
95 + 37.5 = 132.5
Rounding to even means the value is rounded down to 132, so you gained 37.
If instead you had 100 goods already:
100 + 37.5 = 137.5
Rounding to even means the value is rounded up to 138, so you gained 38.
The tooltip on the PP item in your inventory isn't taking your current balance into account when rounding, so comes out differently sometimes. To have it do so would be more overhead for what is really an edge case and where no harm is really done - You're not losing out, 50% of the time you'll get 37 goods and 50% of the time you'll get 38 goods - Try it.
