DOCBOOK-APPS: variable space above 'Table of ...' header

When I use 1.58 XSLs in chain xsltproc -> 1.58 -> fop I get some automated
tables of contents as follow:

first page + abstract
<Table of Contents>
<List of Figures>
<List of Tables>
<List of Examples>
other chapters and appendixes

I noticed that space (top margin ?) above "List of Tables" is different than
in other lists. Is there any clear explanation for it ? Hidden parameter I
missed ? Bug ?

ABX