Hi Stefan,
The scoring / failover aspects of clustering can get complicated. This bug could be one possibility:
http://support.f5.com/kb/en-us/solutions/public/14000/100/sol14155.html?sr=37247153
Also, I see a warning in the manual section "What is an HA group?" which states that you must ensure this is configured on both devices. This tells me that the feature doesn't sync and that perhaps some customers are overlooking this? There also appears to be quite a bit of good information for understanding scoring in general:
https://support.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/bigip-device-service-clustering-admin-11-5-0/8.html?sr=37247045
Hope this offers some help.
Kevin