This is an excellent strip.
I love Lucy's direct, indignant accusation of Snoopy: "You took TWO!" Although nowhere in her counting panels does she say the word "two," which makes me wonder if the spoken-aloud counting is a sham.
If the counting is taken to be real, my explanation is that Lucy has developer her own, personal representation of the number system, which she uses internally and translates when speaking to others. When she's speaking out loud as an aid to counting however, she uses her own symbols.
The question marks (excusing the fact that there are three of them) in the last panel are a bit weird, like he's questioning now Lucy's counting system but his own inability to comprehend it.
Oh, and Snoopy has thought balloons with word balloon tails here again.