Saving a calc spreadsheet as HTML will turn all URLs into absolute links.
Piece of cake to reproduce.

Create one testfile.txt. Create one linktestfile.ods containing a relative URL pointing to testfile.txt. Save linktestfile.ods and also save it as linktestfile.html. 

Open linktestfile.html with notepad and look at the link, it will always be absolute.

No combination of settings will cause the link to be other than an absolute link including disk name etc.

I found this error after debugging a long evening and night in AOO411m6(Build:9775)  -  Rev. 1617669
2014-08-13 09:06:54 (Mi, 13 Aug 2014)
running on Windows 7 ultimate (64).

Checked two older computers running Win XP Home and Pro with OO 3.4.1. The bug exists there too.