Oh yes it's true.

Every [fit for the run] horse running can be a ****** or a Champion depending on the race pace, the track and distance and the barrier and depending on what type of pace suits the horse, the experience of the jockey. Some runners could win with your Granny aboard [on-pacers drawn within 1 to 9 barrier or back-markers from a wide alley] and others need a very experienced jockey aboard [leaders who need an experienced judge of pace over the distance].

Any horse on the day can have a rating from very low to very high, depending on all of the above. Working it all out is the form student's key.
My original thoughts are also the reason that favorites often get a drubbing and long-shots gets up and win!