I think deducting 5% from the Betfair price is not always 100% accurate.

For example, if we had $5 on a $2.00 winner and 5% was deducted from the price it would reduce the price to $1.90. A $5 stake would therefore return $9.50, for a profit of $4.50.

What Betfair does is deduct 5% from the race's profit, not the price. The gross profit in this case would be $5, and deducting 5% from the $5 ($0.25) would produce a profit of $4.75 as opposed to $4.50.

I learnt of this "anomaly" when backing multiple horses but could not manually calculate the profit - it was always more then I expected.

I stand corrected Michael, may I add that I actually pay a lot less then the 5%, but my point still stands.
If you compare the prices there is nowhere an 18% difference, maybe in certain price ranges where the horse has an absolutely minimal chance, then yes.