Explanation:
There are two aspects to any QuarkXPress document:
1.It’s Document version
2.It’s Text flow version

Opening a legacy version document in higher version of QuarkXPress, doesn’t modify its document version or its text flow version.

Resaving a legacy version document in a higher version of QuarkXPress, upgrades its document version to that of the higher version but doesn’t modify the text flow version. The text flow version of the document is preserved, hence there is no text reflow.

QuarkXPress 10 onwards, as we moved from Carbon to Cocoa, we have stopped the support for documents created in versions prior to QuarkXPress 7.0. This included reading of any such documents while opening or maintaining its text flow. It was a huge overhead and cost to preserve and port the legacy code for versions prior to 7.0 to Cocoa. Moreover, Apple had deprecated many of the old Carbon APIs which were used by legacy QuarkXPress versions. Hence, it was a conscious decision on part of Quark to stop the support for documents created in versions prior to QuarkXPress 7.0

Now user needs first to save his documents created in versions prior to QuarkXPress 7.0, in any version of QuarkXPress 7/ 8/ 9, before opening the resaved file in QuarkXPress 10/ 2015. But, since resaving the legacy version document in higher version of QuarkXPress, doesn’t upgrade the text flow version, therefore while opening the resaved document in QuarkXPress 10/ 2015, the application doesn’t have any information about its text flow version. Hence it automatically upgrades its text flow version as per the document version. This may causes the documents to reflow.