Google Search Console Sitemap Indexing

If you wonder why the number of indexed paged if signifficantly different from the number of submitted pages: Keep in mind that the indexed number takes some time to be updated. I've just restructured my sitemaps suddenly showing something like 1000 submitted and 30000 indexed pages for a the file sitemap1.xml. How could it be? The 1000 belongs to the new file where the lines are immediately counted. The 30000 belongs to the previously version. Still it is hard to find out why some pages do not seem to be indexed even after weeks and months - and which they are - or if the indexed counter is showing nonsense again.