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This is because as the reaction proceeds it is getting closer to the 50-50 mixture of R and S components.
Take for example 100 compounds of R conformation molecules. Now one of these molecules undergoes a inversion over a minute and is now a molecule with an S conformation. You now have a group of 99 R molecules and 1 S after a minute. So the rate of inversion could be said to be 1% of all molecules invert per minute. Now look at the percent of the mixture which is perfectly racemic, you have 98 R molecules without a racemic partner and 2 molecules which are perfectly racemic. So, you could say that the rate of racemisation was 2% per minute.
Note: this only holds if you are talking about a pure component which can spontainiously invert into its other form. If there is a mixture of the two components the two forms will be randomly switching between the two forms and at some point the rate of inversion will be greater than the rate of racemisation.
To prove this to yourself take 100 pennies (or any number of random coins) lay them heads up on a table (all sinister) pick any five coins at random, preferably with your eyes closed, flip them over you will now have 95 heads (sinister) and 5 tails (rectus). Do this a few times, if you are truly picking at random you will have picked up at least a few tails coins and re-inverted them. At the beginning of the experiment you were modeling a pure component spontainiously inverting to its other form, which was becoming racemic at twice the rate of inversion. However, as you got closer to a 50-50 mixture (a.k.a. equilibrium) you were still inverting at the same rate but the racemisation was slowing down.
In summery, if you are modeling a pure substance inverting toward its other conformation the rate of racemisation will be twice the rate of inversion, but as equilibrium is approached the rate of racemisation will slow down as the reverse reaction begins to occur at similar rates.