The issue is-- there are lots of way to write down a statement.
One common example is if you're going to internalize or externalize a property of a data structure: eg. represent it with a dependent type, or a property about a non-dependent type. This comes with design tradeoffs: some lemmas might expect internalized representations only, some rewrites might only be usable (eg. no horrifying dependent type errors) with externalized representations. For math in particular, which involves rich hierarchies of data structures, your choice about internalization might can impacts about what structures from your mathematical library you can use, or the level of fragile type coercion magic that needs to happen behind the scenes.
One common example is if you're going to internalize or externalize a property of a data structure: eg. represent it with a dependent type, or a property about a non-dependent type. This comes with design tradeoffs: some lemmas might expect internalized representations only, some rewrites might only be usable (eg. no horrifying dependent type errors) with externalized representations. For math in particular, which involves rich hierarchies of data structures, your choice about internalization might can impacts about what structures from your mathematical library you can use, or the level of fragile type coercion magic that needs to happen behind the scenes.