Well the "⊂" symbol is actually somewhat ambiguous. Different mathematical pedigrees define it to either mean either loose subset or strict subset and its usage

Well the "⊂" symbol is actually somewhat ambiguous. Different mathematical pedigrees define it to either mean either loose subset or strict subset and its usage has never been completely standardised. The "⊆" symbol always means loose subset of, and the "⊊" always means strict subset of. Generally it's best to always use one of those, but in this context, where we're not trying to prove anything, just communicate an idea, the distinction between a loose and strict subset relations isn't really all that important.