How is the Chocobo bonus exp calculated?

It seems pretty random...

Without the FC Chocobo buff you will get 24%, 49%, 74% and 100% bonus.

We had a 3 and 4 person party of level 50's we were getting up to 74% bonus.

Then when we invited a lv 47 into the group we started getting 100% bonus.

The bonus is not consistent, it goes something like this

0%
24%
0%
100%
100%
100%
100%
24%
100%
74%
74%
0%
24%
74%
24%
74%

you get the idea... there's no pattern to it...