Well because they are a lot of methods ,some people buy folowers ,some folow them all day but this its not good as you can seen as a spamer and twitter can block you .But they are also smart people who have experience and they know how to attract people with content
It's not about the quantity, it's about the quality. When I was using that Twando software, I had no idea I was making my account a homing beacon for 2200 Arabic accounts to auto follow me. Twitter started suggesting hashtags trending in Iraq, I had to block thousands of accounts. I added one person to my Niche, and although she didn't buy anything her husband hired me to do their local SEO and Google business page in the United Kingdom! Nice 2 thousand dollar takedown. Moral of the story, it's not the quantity of your followers brotha, it's the quality. If they don't do anything like engage with you, trade/reciprocate retweeting content, wtf good is doing ya?
There are too many services for gaining fast followers. But isn't it better to gain followers by posting something interesting by Twitter?
Yes, but look at how many "interesting" things never get reTweeted or Faved. I think most followers start from somewhere else, such as a website offering, fame, etc. Some might post once every few months, yet will have 12 million followers waiting for that next post.
Follow as many people as you can plenty will follow back. When you reach the follow limit unfollow people that don't follow you and repeat.