That's always been the case. After Win95 came out, every OS revision and Office release changed the default look and feel. It was most noticeable in the toolbars. Started out with visually raised buttons (like regular interface buttons), then the buttons went flat, then the whole toolbar was raised via a gradient, then everything went bonkers with ribbons (meanwhile in browsers you had huge buttons with text below as yet another alternative).

Or the checkmark/cross ok/cancel buttons that were quite common (Borland?).

That "old" look and feel co-existing with "modern" flat/web designs just made the differences more noticeable, never mind the often associated differences in icon sizes and styles.

Not that other systems are better. Linux always had too many widget toolkits (and now those awful upside-down window-title-toolbars), and even Macs always had at least one weird option (e.g. brushed metal or too iOS-like thingamajigs).