Bars and data speeds are 2 different things.
For example you can connect to a wifi network and have full bars signal with that router but if there is no internet/data on that device you will not load anything.
Same thing with this case.
If the towers backhaul is not upgraded or gets a lot of traffic from other users then your speeds will be slow even though you get good signal to connect to that tower.
Hope I explained it simply.
Same thing if you're in a public crowded place. Getting good signal doesn't mean you'll establish a good data connection simply because there's just too much interference.