Fitness index evolution gap between cycling and running activities.

[Rob1733]

I have been using Elevate for a while and I am wondering about the difference in gain on the Fitness trend between cycling and running activities.
I have the impression that cycling activities make it progress faster than my running activities (in proportion).

For example, for a cycling activity of 3h20 (94 km) HRSS 99 my fitness increases by +3.9, for a running activity 1h04-12.8km) HRSS 49, it increases by +0.7.
I find the difference in proportion significant.

Could this come from my parameter settings (zone settings)?