Slitherine

I was trying to upgrade my Recon unit, an SdKfz 222, which takes 2 slots, to an SdKfz 232 8Rad, which costs 3 slots. But when I select the latter as the upgrade, the Total Cost is shown as 2 slots, instead of 1 slot (3 for new unit minus 2 from old unit).

However, if I try to buy a new SdKfz 232 8Rad, the slot cost is 3 slots. Shouldn't the upgrade cost only 1 slot? I'm playing with the Panzer General trait, but I assume that the 25% discount applies only to tank units, not recon units. Or is there some weird rounding going on?

(For some reason, I'm unable to upload any images to show what I'm referring to, so hope the description is clear enough.)

Do you overstrength?
A 2 slot unit can be overstrengthed to 11 and still be 2 slots.
A 3 slot unit will take 4 slots at 11 strength.

Ah, yes. That must be it. Thanks!