Part of the source of the discrepancy is the planned mission is basically just what they get initial funding for not everything they would like to do and the entire mission length they'd like to use. Because launch costs are such a big part of the costs of ever replacing a Hubble or a Curiosity it makes sense for them to last longer than they're funded for because most likely they'll continue to get some funding a long as there's good science and the equipment is still usable. Also the cost of failure is pretty high, if you have something break early there's not another rover/satellite program there to replace it the science just doesn't get done until the next program in 10 years gets sent unless someone else was sending a similar device to that destination.