XMC1400 Time Measurement Jitter Problem

Hello,

I want to measure time with the XMC1400 BootKit.
The timer modul CCU4 is started and stopped by external events.
If I use the internal oscillator DCO1 (48 Mhz), the jitter of the measured counts increases.
At 50 microsec the jitter is under 5 counts, but at 200 microsec the jitter is already around 15 counts.
The jitter of the counts increases nearly linear with the measured time.
I know that using a crystall oscillator solves the problem.

But which effect causes this phenomenon ?
I would expect that the jitter is constant.

Thanks