Relative reference changed to absolute

Why are formulas with relative references to other sheets automatically changed into absolute when closing and reopening Calc files?
Same problem in L.O. 422 and 502. Using Fedora 20.
Only happens with Excel 1997-2003 format files but not with ODS format.
Found similar issue in a closed (qubit) case but couldn’t find explanation (“Saving file changes relative to absolute”).