... which surprises me since it is pretty mediocre picture.
There's no telling what people want in this business. I got an extended licence through IS on a picture of some ends of blocks of wood, a picture I took on the spur of the moment when I was driving and stopped the car for a break.
Why did the guy pay $150 for the photo at iStock if he could have got it at FT for $10?
Who knows? The licence conditions could be different. The buyer may not know about FT. He could be on a project with a paying client so he's not bothered about the difference between $10 or $150 In fact, if a client is footing the bill ,the buyer may even prefer to pay $150. Even though they may be exactly the same images , the $150 makes it seem like a rather more special image than the $10 version.
My advice is to keep your options open. Try to sell your images wherever and whenever you can. It's all money.