I have some knowledge about GPS and it is quite plausable that they had problems - many things affect GPS accuracy, from satellite constelation (how many satellites you can "see" and that could be quite difficult in Monte Carlo due to terrain configuration) to high buildings, tunnels and trees (in surveying there are DOP number that represent signal fallout due to constellation and obstacles - the higher it is, the worse is accuracy).
There is also possibility for some interference with lots of devices on small area. And F1 cars are fast moving vehicles, so in my opinion, you can easily miscalculate exact advantage via GPS on such track as Monaco. Add some possibility for malfunctioning GPS and you have all things together