In many cases, one finds that people who manage thier bandwidth closely to
limit the amount they buy sometimes do strange things (I'm not saying
invalid or wrong things under whatever circumstances, it may be perfectly
valid) to optimize the use of thier current bandwidth.
For example, take a customer who has two T1 circuits from different
providers. In many cases if there is a major difference between the two
upstreams, (i.e. a tier one provider versus a regional ISP), BGP will tend
to select one for most of the traffic. So, in many cases shortcuts are
taken such as prepend all of so and so's traffic through connection A, or,
perhaps in your case, prefer connection A for /16 blocks or larger, and B
I'm not commenting one way or another on the validity of this, as in many
cases it's a business decision versus a technical decision. I'm just
commenting that I've seen this many times.
What you really need is a cut and paste of a few sho ip bgp blahs and sho
ip route blahs....