Affixing Bug(?)

I noticed some inconsistencies when I was trying to upslot a 1 slot Arks Fever unit today.

I did Arks Fever (1 slot) + Junk (3 slot) = Arks Fever & Junk A both at 100% success rate. This was the case for me on two tries, resulting in an Arks Fever (2 slot).

Now on to my third attempt to upslot another Arks Fever (1 slot) + Junk (3 slot) resulted in Arks Fever & Junk at 95% success rate.

Nothing changed, but the success rate changed. 10% affixing boost is still on, I didn't use extra units the first two times nor did I use any success boost items.

According to the affix simulator, 95% success rate is supposed to be correct. Then how did I upslot on 100% twice with generally the same items? I don't believe the rarity of the unit affects success %. Has anyone else had this come up? Or am I missing something?

In the first two affixes, were the fodder units duplicates of the unit you were trying to affix to?

@AndrlCh That could've been the case! Thanks for the reply. That part went over my head since rarely do I actually think of that when affixing expensive units.