.............why do so many reserves win .........

From the time you enter up you should be ready.

To be fair, they would have only known at declaration stage that they were only reserve. They wouldn't be doing much between then and race time. The problems would only arise if they kept missing out on the run, when they are ready for and need it.
Gluts of winners can happen in any cycle or short term spell, but long term there would be no reason for a reserve to consistently out perform its real chance.

There is a logical answer.
Reserves don't have to turn up because of the uncertainty when they do show there is often a good reason,strongly fancied.

If a trainer has a non runner he has to inform the three reserves by phone.....if a reserve goes racing the trainer knows he is getting a run.

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