In years gone by, Sony Ericsson would have all the extra bits to load on when creating a build for the operator. Certainly the operator didn't do it on its own. I am sure the others, like Motorola with its 'flex' software did much the same.
That would mean, say, Vodafone giving Sony the necessary boot logos, wallpapers, ringtones etc. The operator would then have to approve the update, which could take some considerable time as I'm sure we all remember. Meanwhile those with a SIM-free phone probably had their update months before, or even got updates that operators had stopped bothering with.
In more recent months, or even the last year or so, it seems the priority has actually been on getting operator branded ROMs out before the generic versions - so those who buy SIM-free are quite commonly finding themselves last to get updates. I can see why this makes sense, even if it seems rather illogical as it's the version with the least effort to produce.
Without the likes of Xda and flashtools, I'd probably be way behind on firmware updates as I never buy operator branded models!