Agreed with the fact that you should at least try to ask the fabric clerk to write it down at the full price and then you can get the 40% off. It is a little sneaky, but the best way to get that extra 10%.
That said, their fabric is generally a reasonable price so the 10% difference per yard is generally going to be less than $1 for most of their fabric.
I try to use the 40% off on larger stuff honestly where I get a better discount (more than an extra 10%).
I do understand why they don't want to do 30% off and then another 40%. Say a yard is $10 (to make it easy math), the 30% would take it to $7 a yard and then an extra 40% (off the $7) would take it to $4.20 a yard! They'd have to markup fabric to $10 a yard to get to that price that you normally get on their lower priced fabric already.
I am afraid if too many people complain about the 30% off being their regular price that they will just stop offering the 30% on all and just give you one 40% per purchase. Now that would stink for those of us who buy multiple pieces of fabric on a trip! I'd rather get them all at 30% off than 1 at 40% off!