To test, if object cache is working – plugin will set small value at the beginning of the benchmark execution. Later, when it gets to object cache test routines – it will try to read that value. If value does not exist – plugin will make a conclusion, that object cache is not working correctly.
For me it is impossible to say, why quite often it fails – but to be honest – I think something somewhere is not working as it should, unless my assumptions are wrong.. Object caching with Redis is tricky on shared hosting, except when you have a dedicated Redis (e..g through containers). Docket cache is not so bad, but as it uses filesystem – it’s not that fast.
My personal opinion about object cache is quite doubtful – I haven’t see significant improvements with it, at least not on small sites.
