I remember an ATO telling me once, when doing my instrument rating, that the data from the METAR is older than 15 mins when you read it so thats why you couldnt use it to take 100ft off the minima.
I am pretty sure this is the reason.
I think the information is all generated by the same weather station, it just has to do with the fact that METARs can be older than the live information that the station is broadcasting.