Turns out that this was related to an always-on-top window (that I didnt notice cause almost moved off screen).
The always-on-top window (here was 'Everything' search window) always shows first in the AltTab 'dialogue' - then comes the window with focus.
Most software will AltTab as normal -- focus is already on third window when AltTab pressed, but with some other software (so far: SC & PhotoSupreme/IDimager), the Everything always-on-top window is read as having focus, and AltTab pressed once moves focus to the second in line i.e. the app itself, (then you need an extra Tab press to 'get away' from the always-on-top window to the third window).
Unimportant, but interesting, (says Tom procrastinating :p)