Just had a thought about Merc. I wonder if there is some sort of a deal where the team will not 'disadvantage' one driver by strategy. So they had to pit both cars at once. It does seem stupid that with Ferrari showing good pace, they would put all their eggs in one basket and pit both cars at same time after safety car.
Am sure they mentioned last year that both drivers had to do the same number of stops each race, but could choose what tyres they wanted to use.
It does seem daft, unless Merc knew they could never do just two stops, but were caught out by Ferrari who thought they could (and did!)