This is the count of 1023 bits numbers [(x - x/2) = x/2]. 0 is a valid leftmost digit.
It matches the common-English usage of a phrase like "a six-figure salary." A salary of $020,000 isn't what's meant by the phrase.
You can verify this by, say, running `openssl genrsa 1024 | openssl rsa -noout -text` a few times and looking at the generated prime1 and prime2. They each have the 512th bit set. (They seem to be printed with a leading hex "00:", but there are 512/8 = 64 bytes afterwards, and the first byte always has the high bit set.)