Looking at Freeza's statement, it seems to be implied that Freeza is talking about Nail losing ki because he had his arm ripped off, and that he couldn't regenerate that stamina/physical strength back (from losing the arm) like he could his actual arm.Freeza: This is a surprise! You can regenerate? However, even if you return to normal, it’s the same thing. What’s more, it seems you can’t go so far as to regenerate stamina [alternatively, physical strength]. Your battle power has dropped from what it was before…
In Piccolo's case, after his arm had been blasted off, the damage had already been done at that point. If regenerating an arm doesn't cause one to lose ki like it's implied above, why would he not regenerate it? Raditz stated "Making secret plans, little friends? Well, if that's how you choose to waste the last moments of your lives...enjoy yourselves! Wa ha ha!" So it's not like Piccolo didn't have time to regenerate his arm. It appears to only takes around 5 seconds anyway. Surely there must be some sort of power decrease when you regenerate your arm, otherwise there is no reason for him to not grow it back until after the fight.
Like I theorized before, maybe regrowing an arm uses up a good bit of ki. Piccolo stated "Quite...an inconvenience...Having to fight...with one arm..." Does this make sense to anyone, because it doesn't to me. He doesn't have to fight with one arm, but he chooses to, probably because he's already thinking about using the Makankosappo, which he mentions a few panels later, which is implied to use a lot of ki. So using up more stamina would just be pointless. Regeneration even sounds like something that would use someone's ki. I can't see something that useful not costing ki (apart from Boo).
Maybe both losing a limb and regenerating said limb cost a Namekian ki?





