I think these are sully generated by that journal or publisher so they calculate the number of published manuscript divided by the number of submissions. This may be an indicator of the quality of the journal or how hard it is to get “in” - they seem to say look how few we accept of the ones that we receive.
To me, that also means if the % is low, please do not be disappointed if your paper is not accepted.
To determine a journal’s acceptance rate, the number of accepted manuscripts is simply divided by the number of submitted manuscripts. For example, if in one year a journal accepts 60 manuscripts, but 500 are submitted that same year, the journal’s acceptance rate is:
60/500 = .12 or 12% acceptance rate
Additionally, some journals calculate their acceptance rate differently. For example, looking at the number of accepted manuscripts divided by the sum of accepted and rejected manuscripts. In this other approach, the publisher is reporting a lower acceptance rate than a publisher with the same number of accepted and rejected articles. You can see how that looks below: