After takeoff, why do some airlines' flight attendants rise from their jump seats earlier than other airlines'?
The industry standard is to switch the seat belts signs off while crossing 10,000 ft. And this is usually the moment when flight attendants get up. But really depends on company policies as you saw. Even within the same airline there may be different standards for different fleet, which actually depends more on timing for service that on safety.

In any case, the decision to let the flight attendants to get up depends solely on the captain, who is the one looking after their safety. If he suspects turbulence ahead he will delay the signs as much as required. 

You have also to consider that flight attendants receive special training on how to move safely around the aircraft, even with severe unexpected turbulence. If you encounter some light turbulence in your next flight pay attention on how they move: looking for a stable position opening the legs more than usual walking and pointing their hands to places they could grab immediately.