Occasionally I'm puzzled by inconsistencies in the orientation of
thumbnails. Here's an example for one particular photo:

Browsed in Thumbnail Displayed when opened
---------- --------- ---------------------
Thumbs Plus Correct (portrait) Correct (portrait)
IrfanView Correct (portrait) Correct (portrait)
PaintShop Pro Correct (portrait) Correct (portrait)
EXIFer Incorrect (Land.) Correct (portrait)

Can someone explain why that variation might arise please?

(Note: I use the word 'correct' above, but I suppose 'as expected' or
'intuitively correct' would be more accurate.)

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Terry, West Sussex, UK