How do Fortinet Firewall Lag's load balance

Hi guys

We have looked at some Sevone graphs and noticed that on a Lag connected to a Cisco router, that we have a 50-50 spilt of traffic from the Cisco to the Fortinet, but in the other direction we don't see the same pattern.

We have a Lag made up of 2 x 10 gig links, and have 75% of traffic on one physical link and the remainder of the traffic on the second. 

Having looked at the config of the Lag, we don't seem to have anything set in terms of what hashing algorithm is used, so assume we have the default set.

Can anybody explain this behaviour or explain to me what options there are in terms of load balancing traffic across the lag's are?

Thanks

James