Uh oh. That's actually the wrong Basic set book. You're using the Basic rulebook for the the BECMI rules, written by Frank Mentzer and published in 1983. Were using the B/X rules by Tom Moldvay and published in 1981. Looks like this:
The rules are
pretty much the samesies, with only a few notable differences. The main difference between the BECMI rules and and the B/X rules was Moldvay's B/X used only 2 books: Basic (covering characters levels 1-3), and Expert (covering characters levels 4-14). Mentzar's BECMI system had five books: Basic (levels 1-3), Expert (levels 4-14), Companion (levels 15-25), Master (levels 26-36), and Immortals (everything beyond 36)
For the most part they're pretty similar, but there are some notable differences besides how high you can level up. The BECMI Cleric started out with a spell already at Level 1, whereas in B/X you don't get spells until Level 2. The Thief Class actually had some major differences between the two editions, and honestly, I think most people argue that the BECMI thief class got seriously hosed over compared to the B/X thief, because a 14th Level Thief in B/X has about the same Thief skill numbers as a 36th in BECMI. It just took way longer to become awesome at Thief class in BECMI and your skill improvements were pretty minimal from one level to the next compared to B/X. At first level, all your numbers should be the same, but I'll double check.