That's not a question I thought I'd be pondering coming into this season. The consensus was the opposite, that Bess and Hartline would make up one of the worst receiving corps in the league after we traded Brandon Marshall. But that hasn't been the case. Let's look at their standard stats.
Hartline: 29 catches 514 yards 17.7 yards per catch 1 TD
Bess: 22 catches 346 yards 15.7 yards per catch 0 TD
Hartline is leading the league in yards and Bess is 19th. Hartline is 8th in YPC and Bess is 18th. They aren't catching many TDs. But they are really racking up the yards, and with a rookie QB throwing them the ball. So by standard stats they are doing very well. The advanced stats show them in even better light.
Bess leads all WRs in win probability added. Hartline is 16th. This is significant because win probability is telling us how much a player is helping his team win. Brian Burke explains it here. Long story short, Bess is ranked so high because he catches passes in important situations, such as 3rd downs. Hartline is lower because even though he has caught more passes, those plays haven't come in as many situations that increase the team's chances of winning the game as Bess has. Still, Hartline is doing very well by the WPA metric.
Even Dolphins fans are having trouble understanding what this means. They have seen Bess and Hartline play for a few years and not produce like elite WRs. So when they see that they are rated so high by these stats they think something is wrong with the stat. Win probability isn't telling us that Bess is necessarily the best WR in the league (at least by conventional standards). It's not telling us him or Hartline are the most talented WRs. It's just telling us how effective they have been at making plays that help the team win and avoiding plays that hurt the team's chances of winning.
There are a few biases I see in regard to Bess and Hartline that prevent people from accepting what the stats are telling us. Hartline's white, doesn't run a great 40, and isn't physically imposing. He doesn't look like a stereotypically good NFL WR. Bess is similar. He doesn't run a great 40 and isn't physically imposing, like a Calvin Johnson. Combined with how they look, they don't run the flashy routes that show up on highlights. And finally, they haven't generated the eye-popping stats that the stereotypically good NFL WR gets (>1000s yards and around 10 TDs). So based on what we are told by the sports media, they don't look like what the best WRs are supposed to look like (both physically and on paper in regard to their production).
Until now, I think a lot of their lack of elite production has been due to the fact that they haven't had good QBs throwing them the ball. Wes Welker was similar to them during his time in Miami. But Welker didn't all of the sudden develop new skills in NE. To my knowledge, he didn't get any faster and his hands didn't get better. The difference is that in NE had a good QB and he was getting thrown to more often. The latter is often overlooked. Currently, Hartline is 7th in total targets (passes thrown to him). Bess is 27th, and that's with a teammate getting the 7th most targets. So they are both being thrown to a lot, whereas last year we threw to Brandon Marshall a ton, 8th most in the league. While Bess and Hartline ranked 52nd and 63rd last year.
So the reason they are producing so well this year is due to the fact that they are getting more passes thrown their way and they are performing efficiently (catching a lot of them) when the pass comes to them. Some are having a hard time believing they are producing like elite WRs because they have never done so before and they don't look like the typically elite WRs (aside from Wes Welker). That's not to say Bess and Hartline (or Welker) are as physically talented as someone like Calvin Johnson. But they are producing like them. And that should tell you something about how, in general, we evaluate WRs and value the skills we covet in WRs.