I'm sure someone will correct me if i'm wrong but I work for an Ambo Operator and this is how I understand it (I'm hazy on it because i'm well over that minimum so doesn't really concern me, misc information dump).
750tt for SPIFR under Part135 is the legal minimum. 1000tt is a preferred requirement put in place by the DHBs (I am unsure who the genius is that pulls these numbers out or where exactly they pull them from).
Generally the DHBs will NOT award contracts to operators that don't have pilots who meet their preferred minimums, they simply go to the next air ambo and pay a premium from the tax payer kitty to get what they want (or have been told by someone somewhere along the line that they need to have). So as far as the operators are concerned these minimums are a must.