Is it always true? When I worked for a grocery chain, I was part time but because I averaged over a certain threshold of hours per week that was less then 40, I

Is it always true? When I worked for a grocery chain, I was part time but because I averaged over a certain threshold of hours per week that was less then 40, I received full time benefits. This might have been a union requirement rather then a legal one but it worked out the same for me regardless of whose requirement it was.