The ratings are always nebulous at best. Most of them should be cut in half for practical use IMPE but it depends a lot on the brand.

The ASM G3 advertises that it is good for a 250 gallon tank (6" column, 24" height, sedra 5000 motor) but the same sized euro-reef skimmer (RS135) is only rated for 135 gallons. I'd say that the Euro-reef rating is probably more accurate for that size skimmer. The Tunze published ratings are accurate for what they are meant to rate - low sensitivity tanks. If you are planning on keeping SPS (high sensitivity) or SPS and an "above average" fish load then Tunze recommends cutting the skimmer rating by 40% or 60% respectively. So the 265 rating is probably more like 120 or so for most tanks.

Anyway, you need to take this into consideration I suppose. Saving a little bit on a skimmer now and then having to buy a bigger one later is an expensive proposition.