Green’s Dictionary of Slang

1. to assess, to analyze.

1936

R. Chandler ‘Goldfish’ in Red Wind (1946) 168: Sunset studied me with his close-set expressionless eyes, a little at a time, but very thoroughly in the end, adding it all up.

2. to make sense, to work out as expected; esp. in phr. it all adds up, or it doesn’t add up.

1941		B. Schulberg What Makes Sammy Run? (1992) 61: You’re shell-shocked and you don’t add up.
1951		D. Stivens Jimmy Brockett 30: It didn’t add up though with what I’d heard about these artists, because though she had come around to have a drink in my flat, I hadn’t even been able to kiss her.
1958		E. Gilbert Vice Trap 134: It adds up, Nick. [...] It’s a scam deal.
1997		C. Hiaasen Lucky You 20: It don’t add up.

3. (also add up to) to amount to, to signify.

1958

R. Prather Scrambled Yeggs 32: ‘What does it add up to?’ ‘It adds up to something big.’.