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Yes, but there are very stiff steel frames and not very stiff steel frames.
From the outside and from the inside there's not much difference in the way they look,
So where does the stiffness come from?

1% small differences in section/butting
1% small differences in geometry
1% small differences in lug form (in particular, the BB shell transfers some high stresses from tube to tube, a small deflection there is highly magnified at the other end of the tube)
97% in the imagination of people riding the bike rather than actually measuring its properties.