Yes, both of you have got it - the S-S is the odd one out.
Well done, indeed, Dan,
for identifying the pieces so quickly! You actually had slightly different ones to me. I originally went for Mendelssohn’s Scottish, so that I had three 3rd symphonies with a national title. I had also forgotten about the Saint-Saëns Suite, but went for the 5th piano concerto instead, but the Suite algerienne will do for me:
Tchaikovsky – Symphony No.3 ‘Polish’
Saint-Saëns – Piano Concerto No.5 ‘Egyptian’
Stanford – Symphony No.3 ‘The Irish’
Mendelssohn – Symphony No.3 ‘Scottish’