I think
tu154 hit the nail on the head! PPLs and CPLs are not bright enough (that's me included BTW!) to be able to cope with a "difficult" number like 1.8 and therefore we are only allowed to deal with 1.5 deg/1,000 ft.
When we grow up into ATPLs, then we can use the slighter higher number
Alternatively, maybe global warming has changed the rate and some books are a little out of date!
Cheers
Whirls
PS - the last bit is NOT to be taken seriously!