Yes, interesting, isn't it?
The other point worth noting, was that the original concept for Starfleet rank in Trek was that the grades had more to do with identifying the
role or appointment in which the individual was serving at the time, rather than progressive steps of substantive
ranks in the way we currently understand contemporary military rank structures -- hence Kirk moved from the role of Chief of Starfleet Operations to the role of Starship Captain for the duration of that mission and so his role insignia changed accordingly.
The TMP novel strongly suggests this and it's very much tied in with the idea of Starfleet
not being a
military service -- but much of this concept has been lost since. I suspect that this is mostly to do with successive producers, writers and the audience not having an easy frame of reference for this less rigid role-based concept and therefore the tendancy is always to default back to what everyone is familiar with, which usually ends up being "
oh, it's just the Navy in space" -- and this then quite frequently morphs even further to become "
but the USN does it such-and-such a way, why aren't Starfleet doing it like that?" Well, because it's Starfleet and it's set hundreds of years in the future in a Federation that involves multiple other interstellar civilisations so they can do it any way they want...!
I mentioned this near the start of the General Star Trek: Picard Discussion Thread so please forgive me quoting myself:
Given the nature of Starfleet's organisation and the variable attention it pays to it's military role, I guess it's fair to say that appointments to various flag-rank positions are likely just that: appointed roles rather than substantive ranks on a fixed ladder in the way that we would understand them now.
Picard could well have been the most senior captain in the whole of Starfleet when he moved on from the Enterprise, probably with substantially more command experience than many junior admirals. It's not beyond the realm of possibility that under Starfleet's system this may have been more than enough to make him eligible to move straight into a three-pip appointment and then he was subsequently re-graded for whatever came next with his big mission.
From what has been described so far on-screen, it seems that Picard apparently went from the commanding the Enterprise to four-pip admiral commanding the Romulan rescue efforts (we know it's four pips 'cos we've seen his uniform, but equally it's a comment by a reporter, so it may not cover the full and accurate facts...!). Nevertheless, this is probably still the easiest way to explain, in-universe, how the senior Starfleet appointments work and avoids the need for a lot of convoluted fan-w*nk explanations to retroactively justify what are, really, just out-of-universe creative decisions made by the production team. At least in Picard's case it kinda makes sense: he has all the leadership experience, he's very much a hands-on operational commander and he has the links to Romulus so, overall, he was probably the most qualified person to take on that role.