Here's our output, for comparison. Seems to be a little more evenly distributed than yours, but e3a takes the lead by far.
bat> netstat -i Name Mtu Network Address Ipkts Ierrs Opkts Oerrs Collis Queue e0* 1500 none none 0 0 0 0 0 0
e3a 1500 144.212 bat 935492178 17 1070359540 268973 0 0 e3b 1500 144.212 bat 712967233 17 800877896 233521 0 0 e3c 1500 144.212 bat 527507321 2 669191765 212388 0 0 e3d 1500 144.212 bat 522712811 89 672437298 201463 0 0 lo 1536 127 localhost 30198 0 30198 0 0 0
---
How much bandwidth are you using there, I am only barely using 110-130Mbits average.
Im thinking it hasnt reached a 'threshold' to fire up more links.