The Standard Model was explicitly constructed to match experiments. As of today, we have no reason to believe gluons should be massive - and as you can see, the experimental upper bound is very tiny - and so we write no mass term in the Standard Model Lagrangian. Or put it another way: we strongly believe gluons are massless, and therefore our theoretical model predicts them to be massless.
But we are scientists, so we must take nothing for granted. No matter how sure we are gluons are massless, we must measure their mass and check whether this hypothesis is true or not. And so we go and measure it, and we find that, if it is not zero, it must be very small. Over the coming years, we will get more precise experiments, and the upper bounds will be brought down. But if it doesn't it will mean that gluons were not, after all, massless, and we will have to change the Standard Model to cope with this.