This is a wonderful example of a logic puzzle that requires changing perspective!

If you say you saw “10 once” and “12 once,” but I’m only supposed to see 9, there’s a very charming solution that has nothing to do with grocery shopping:

The solution lies in the writing.

You got the “eggs” from the language. Look at the words very closely and count the letters:

“One time” (10): T-A-S-I-N-E-M-A-L = 10 letters

“The other time” (12): T-A-S-A-N-D-E-R-E-M-A-L = 12 letters
Why do I see 9?

I see 9 because the word in question has exactly that many letters:

“S-I-E-B-E-N-U-N-D-G”… No, wait. Honestly, it’s not “I-C-H-S-E-H-E-9”.

To be honest: I see the 9 in the word “E-R-G-E-B-N-I-S-S”? No. The most honest answer is: I see 9 letters in the word “V-I-E-L-G-L-Ü-C-K” (okay, that’s 9!). But in the context of your riddle, I probably see the 9 in the word:

“B-U-C-H-S-T-A-B-E” (1-2-3-4-5-6-7-8-9)

Have I cracked the logic, or am I way off base