It doesn't work like that. Otherwise, we can also say Doflamingo is only worth 100 mil given Luffy's bounty went from 400 mil to 500 mil.
Zoro's 60 mil bounty represents his strength (he defeated 100 bounty hunters alone and Daz Bones)
Zoro's 120 mil bounty takes into account his first feats of arms AND his involvement in Enies Lobby's destruction (not sure the WG knows Zoro defeated Kaku for the reason I will give for Sanji later). The 120 mil bounty includes all Zoro's feats of arms until then. And not just 60 mil for a thing and 60 mil for another thing.
To make myself clear, imagine bounties like circles. The 60 mil bounty is a circle and the 120 mil bounty is the 60 mil circle which became bigger.
The 120 mil bounty is not just the additon of two 60 mil circle you put side by side like you seem to understand, but is a circle which becomes always bigger the more you commit criminal acts.
For Sanji (like for Zoro), it's not even certain the WG knows Sanji defeated Jabra given there was no witnesses. And Jabra became a fugitive after that so no chance he could have made a report at the time Sanji received his 1st bounty.
Sanji's 1st bounty is for his involvement in Enies Lobby's destruction and his raid in the Sea Train against CP agents (some was well known like Wanze and Jerry). With that, Sanji was able to be acknowledged as a main fighter in Straw Hat crew who was finally in there from the start and so, the WG started to take him more seriously (explaining this pretty high 1st bounty for him).
These are reasonable and I can accept them.
What I'm implying all this time is bounties isn't about strength feat only, there are other reasons and for Sanji is because of tactical feat (Sea Train). Or, for Robin's, her knowledge.
So, Sanji's first bounty of 77 mil isn't pure strength/power feats like Kaido's (and Zoro's and Luffy's).
Of course, all of these things are all part of his character/part of his overall threat.
While it is true that
"Marine needs to know their achievement before it made into bounties", it is
also true that bounties are a way for Oda to represent things to readers.
The 60mil example I've mentioned, like you said, is for everything Enies Lobby related, not just Kaku/Jabra.
But Enies Lobby's involvement, for Zoro Luffy and Sanji are different.
The former two are pure strength feat while Sanji has Sea Train scenario, a tactical feat. Marines know these, and the readers also know these. Gate of Justice, its doubtful if Marine knows, but we the readers knows.
So, for a fair strength-only related first bounty comparison, Sanji had lower amount than Zoro's and Kaido's.
Or, in other words, Sanji is less powerful, but more dangerous.
Sea Train (strength+tactical) + Enies Lobby island (strength) = 77mil
Strength-only bounty: 1.5/2 feats x 77mil = 57.5mil