I was surprised the 'a' didn't change the meaning vs simply 'ujmey. It seems like 'uj'a' should be bigger or more precise?
That's right. An uja (or 'uj'a') is nine ujes (or 'ujmey) long. So 50 ujas would be 450 ujes. What made you think the meaning didn't change?
I'm guessing he didn't see the subtle vowel change in the English translations "ujas, ujes".