Regarding to PDF specification file version should be declared at the begining of file like %PDF-1.4 in your case. But later Adobe decided that it can be updated in document catalog (/Version/1.5 in your case). Primary concern of such update - modifting document using incremental update (in this case original file remains unchanged at the begining, all changes are stored as addition after it) and using features that are not allowed by original spec. version (for example Polygon annotation require pdf version 1.5 or higher). If this record is present - it override version specified at the begining of document, so problem is in your "other software" (and maybe in creator, if it use such version update without a reason).
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