Looking on the Jet 2 website, to fly to Lanzarote, I put in an enquiry for a price for 1 traveller and it came up as £159 return,but if I put in 4 travelling the price increases to £189 return,or 8 travelling £249 going out and £189 coming back!
Now if I buy one pair of shoes for £20,would they increase the price if I buy 4 pairs!!
Might be worth bearing in mind when booking for more than 1 traveller.
I dont know if any other flight operators do this, but could well be worth checking out.

It means that there was just one seat left at that price. I know that Aer Lingus always tells you if there are only one or two seats left at that price, not sure about the others