I've been saying this for years - refs won't call a travel unless they're 100% certain, and honestly, that's how it should be. These officials are sprinting backwards while tracking world-class athletes moving at full speed. The human element is real.
If refs called every questionable step, we'd have whistles every other possession. The game would be unwatchable. I'd rather see them let the borderline ones go and only call the obvious travels.
This is why I always tell people to give refs some grace on these split-second decisions. The job is harder than most fans realize.