Ok , it seems I got it.
For sure this time
Max number of entries in a partition in this case is 300
(it depends on yr preferences of tag browser - you can see that in my first picture)
thats why calibre created 3 partitions
last two are with no rating
first one has ALL ratings
the only way to see them visually (except tooltip) is by looking at the head and shoulders figure left to the author name
it's not really the best solution