I don't know about SEALs but grunts in Vietnam came up with some "different" guns:
I've seen 2" Model 10s, Smith 357s, Browning HPs, 44 Mags.......all sorts of weaponry.
The truth being, be cause they had them, doesn't mean the whole unit had them or they were issued. It means they had their folks send these guns to them. They weren't authorized but I never saw a commander in a grunt until that cared. If they wanted to pack the weight, and the unauthorized weapon gave comfort to the carrier they said nothing.
I thought about it my self, but failed to see any of those doing anything I couldn't do with my issued M1911A1.
I think now its harder to smuggle personal weapons into a combat zone. In some units it may be easier then others.
When I was in the NG I refuse to accept the Berretta, so when I carried, I carried my personal M1911a1. Not many knew it wasn't authorized, those who did, didn't care. All that really mattered is when I qualified, I shot expert. The real benefit (in peace time) was I didn't have to deal with power hungry armorers. Same with my rifle, I didn't use (in the Guard) a M16a1 like every one else, I used my Colt SP1. Again those who knew the difference didn't care.
In short, if you saw someone carrying a Brand X revolver in Y Caliber, no way indicates the Unit as a whole carries them.
Even as in civilian LE I tried several different guns, as long as I qualified with them it was ok. But I always ended up coming back to my issued service revolver, the Smith Model 28.
But people still go for the "cool" factor, whether they are SEALS, grunts or cops.