We have a Marathon G3 that sits/sat on top of the anchors in both our current and old car--I think it's just a small car thing. I'm used to it so it's never struck me as odd.
With the G3 (and I assume G4 too?), you can thread the latch straps up through the seat (pull them through the openings where the harness belt attaches) and tighten that way--I find it actually works much better for tightening even in cars where the anchors aren't under the seat. If you ever have to do it again and want to use LATCH, my usual way to do it is to loosen the straps so I can pull the seat out enough to reach the LATCH anchors, buckle it in, thread the straps up through the seat, slide it into the correct position (covering the anchors), and then tighten. To release it you do have to loosen the straps first so that you can pull the seat out enough to reach the LATCH anchors to unbuckle.