It is due to temperature change, the X+ is a reasonably aggressive file for the stock turbo, you could try a different N75 valve to reduce spike (I presume you are running the one intended for you car/that file), but I would just install a boost controller as you will need it anyway once you upgrade your turbo ( unless you get an APR ‘kit’ but I’d still get at least an MBC and limit boost to desired PSI level) the spike doesn’t really help you anyway
On stock turbo I consistently ran 14.4 to 14.2 @ 96-98mph (2002 Jetta full weight no rear seats, kept the spare on 17 inch tires) with an Upsolute chip/3" dp and N75H valve, when it got colder my car would spike higher. With just a chip (no other mods) I got 180whp and 240wtq