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Full Idea
Mathematicians use the 'abstract/concrete' label differently, with arithmetic being 'concrete' because it is a single structure (up to isomorphism), while group theory is considered more 'abstract'.
Gist of Idea
Mathematicians regard arithmetic as concrete, and group theory as abstract
Source
Stewart Shapiro (Philosophy of Mathematics [1997], 4.1 n1)
Book Ref
Shapiro,Stewart: 'Philosophy of Mathematics:structure and ontology' [OUP 1997], p.109
A Reaction
I would say that it is the normal distinction, but they have moved the significant boundary up several levels in the hierarchy of abstraction.