Load sharing across trunk links / LACP

In our main data center we have some Viprion 2100 blades with two 20 Gbps LACP trunks each. Load sharing is src/dst IP address and splitting the outbound traffic perfectly 50/50 across both physical links.

In a smaller Data Center we've got BigIP 4200 with 4 Gbps LACP trunks to a single switch each. Load sharing is also src/dst IP, yet here the load sharing is 70/10/10/10

Has anyone else observed this? I understand that inbound traffic distribution will be determined by the switch, but outbound should be done by the BigIP and makes no sense that they'd be so different.