Exchanges v the SP

Strange one tonight, watching the 9.00 at Chelmsford. Mercury backed into 5-1, from 8-1, on course, but the exchanges fluctuated between 18 and 11, totally ignoring the on course gamble. I don’t follow the exchanges but do others know why that may have occurred? Btw, no after-timing, but I had my £2 on at 7/1.

Happens in Dundalk every week

It would cost an off course bookmaker no more than 2k to shorten a horse from 9s into 5s. If they are saving 40k+ by doing so it makes sense.

Never bet SP
Never read anything into "gambles" on midweek all weather fodder
Judge your bets against BSP not SP