Red Go Go Go
The Oddsmatcher calculator flags these bet matches when the odds at the bookie are longer than the odds at the lay exchange.
This situation means that you will always be able to make a profit. The margin is very low (because the odds are not usually far apart) but if you have a big wedge of money, you can create £££. Deliberately exploiting these quirks in the bookie/lay odds is called 'arbing'. If you arb a lot, with big money, the bookies will zap you. Nonetheless, some people use only arbing to make their matched betting profits.
The chance to arb appears in highly mobile markets with constantly changing odds, where the bookies fail to adjust theirs quickly enough to stay shorter than the lay odds. Horse racing is the most obvious opportunity. And these red flagged matches don't stay favourable for long - bookies will see a flurry of bets on an event and know something is happening. The odds are often only favourable for a few minutes, if that.
As far as I can read, Oddsmatcher only returns 'reasonable arbs' - ones that aren't going to infuriate a bookie too much if they see they've mistakenly gone too long with their odds. We are talking about differences in odds of, say, 5 v 4.9, so you might make 30p or so, instead of losing that 30p. Professional arbers look for greater differences, but risk the bookies pulling the bet because, they argue, it's obvious to the punter that there's an error they are trying to exploit.
There is no reason not to take red matches on either qualifying or free bet. You'll make slightly more profit than normal. But you'll have to be quick to get your betting slips loaded.