I have no idea why they've pitted Webber when they did, he was gaining on Grosjean, his tyres clearly weren't gone, but they've cost his strategy completely. It's a merc/mclaren mistake, costing your strategy WAY to early for a stupid supposed undercut on a car that clearly wasn't going to be pitting for ages.
They've given Webber an extra pit to undercut a car that wasn't remotely close to pitting, so gaining a few seconds at the expense of an extra 22 second pit stop. He will be faster, but would have to pass Grosjean and Vettel which likely won't happen.
If he'd been pushing 3 stop strategy from the start he would have passed Grosjean or pushed him on to a 3 stop himself which would favour red bull anyway.
They are banging on about Vettel/Grosjean gap but if Vettel does another few laps the end of the stint will see Vettel pass Grosjean pretty easily, the speed at which WEbber/Vettel caught Grosjean when they wanted to was easy. They could, if Vettel can get past Grosjean after he pits quite quickly, keep Webber out long and slow and use Vettel to hold up Grosjean and protect him which is possible.