Oh i'm sure there are tons of reasons for why their polls and our polls skew like it does. But when a poll comes out where Cavendish is more popular than all of the above plus Usopp, Brook and Franky it tells you that the people voting have different priorities and taste. Cavendish even edges out Doffy, Mihawk and Croc. Rocinante is higher ranked than them too. And the pink haired Vinsmoke girl is even higher still. Mindblowing stuff
Well, another important factor is the design of the poll. Voting for only one character is not the most effective method to measure a popularity ranking. Hypothetically speaking (purely theoretical), if there were a character who is everybody's second favorite, but nobody's favorite, he wouldn't get a single vote despite being super popular, lol.
On a more practical analysis, one weird consequence of this polling method that I can think of is strategic voting. For example, maybe you won't vote for your favorite characters because you know that they'll get a bunch of votes already (like the Strawhats), so you'll choose a side character of your liking to give him a chance to get a good position. So if you're a Cavendish fan in a war against Barto fans (lol), maybe you'll vote for Cavendish to make sure he'll beat Barto, even if your favorite character is actually someone else.
It's also a way to send the message "please, Oda, use this character more in the future, we love him/her!". That's one of the reasons someone like Reiju may get so many votes.
In the end, many of these deviations end up rigging the truth and some characters may get too lucky or too unlucky.