Toc output

I am formatting a book in docbook5 ns to pdf using
 docbook-xsl-ns-1.73.2 stylesheet and fop-0.94

 in the toc the page numbers are not all in the same column,
 
 in pages numbers 1-46 there is one case with one less space between the last dot 
and the number:
...  17
... 17
...  17
and two where there  are fewer dots
... 35
.  35
... 35

I wander what is happening here
   Ian