Because when you put them on only one or two hands you're likely overestimating your reading ability, unless you've played them quite a bit and they are too simple to mix up their play at all.
The best way to understand it is to read some analysis by multiple people of a given hand and just look at what ranges people put an opponent on given the reads and situation, and also read where people tak about "balancing" thier own range, where sometimes they do one thing and sometimes they do something else, then look at that same situation from the outside and you can see that the range isn't just one or two hands for these players. But, that's not the players at the micros doing that balancing.
btw, Do you play $10nl FR at Stars under the same name? I was sitting a couple tables one day and noticed someone with TAGgier than normal stats and I thought I recognised your name.