Looks like I've been using a quite frankly incorrect ruler for the last 10 years. This might explain a few problems!
Order top to bottom: New BH, Old BH, cheap chinese metal ruler.
0.3% is well within the expected creep over ten years for cheap plastic products, so it is very likely that it was more precise when you bought it.
Temperature-cycling is the main driver of such creep, but other environmental factors, including humidity and VOC air pollution also helps.
If you want long term dimensional stability, you need to pick your polymer carefully, start with ABS: 50 year old LEGO bricks are still on the mark.
PS: No, you can not buy Invar rulers: 36% nickel would give you allergy in no time.