This subject was raised recently in SLF forum, with specific regard to certain LoCos.
Some research was put forward that squashed this urban myth. That said, all airlines now review their prices all the time. If there is one seat at (say) £99. available and you check back two minutes later and it's been booked - you might see £109. It's the correct price for that minute and that status of that flight.
If, at the end of that day, the computer programme says, "Not enough seats sold - lower ten more seats down to £99." Than that also will be correct. Prices are varied for everyone but not varied for you specifically.
That is not to say it's not happening somewhere ...