Hamilton contract up this season though working out the last few points on a new one. It wouldn't surprise me if he delayed the reading through and adding stupid new lines that need to be checked again and again for a few races just to check Merc were still looking dominant. You might after two races have not wanted to instantly commit yourself to Merc for a few more years if Ferrari were just about to start a dominant period and your own contract was up at the end of the year... as is Kimi's I believe
Realistically it's just stupid long contracts and little changes get made over one point, or a point added and the whole thing needs to be read again... lest he get stuck with a line he missed saying Merc will be allowed to suspend him from a cable and lower him on to a stage for a PR event... as he had to do at Mclaren.
Rosberg signed last year to extend by 2+ years. Sounds like he's there till end of next year with an option for more.