All N tracks are not created equal

[Biased turkey]

I have a Fleischmann steam era digital starter set. The included tracks have a plastic ballast , they look very similar to the Kato unitrack.
Just to compare, I purchased some Atlas tracks, 2 turnouts, one MRC DC pack, one Life Like GP18 and 2 40' boxcars and made a 2nd layout with that.

The Fleischmann loco and cars run nicely on both the Fleichmann and Atlas layouts, but the GP18 and boxcars derail frequently when negociating the turnouts on the Fleischmann layout.

Could someone try to explain why?

Both Atlas and Fleischmann are code 80 and I thought that every N scale tracks manufacturer follow the N scale standard, but obviously some don't.

 

[umtrr-author]

The tolerances are fine enough in N Scale that it wouldn't take much of a difference in the turnouts, wheel gauges or both to cause derailments.

I would check the usual suspects first: wheel gauge and track gauge. I'm not much help on the Fleischmann track since I'm not familiar with it.