Please, add the -n parameter to the diskspd command line when 1 thread benchmark is started;
this is the way one thread applications run;
otherwise the benchmark will be set to run on the CPU 0 (default affinity)
This may be what the original poster found.
This happens only in Windows 10, single thread test and the size of read/write is small. e.g. a lot of random synchronous read.
In this case, set the affinity in fact increases the read/write performance.
A closer look at it, the average time of Readfile()/Writefile() is shorter with the affinity set.