This is really hard to answer. I'm going to say that in this sense, "offering" is a "term of art". That doesn't mean it has to do with art, it means it's simply a phrase that's used in a particular field or area.
I say this because in the field of economics and finance, one tends to see the term "offering" used with financial products. "An initial public offering (IPO) of stock", a "stock offering", a "rights offering" a "corporate offering" etc.
Actually, the more I think about it in this context, the more I feel like there *is* a difference:
"A differentiation strategy is one in which the product OFFERED is differentiated from the competitors." = the strategy is an on-going, continuous, long-term strategy to make and keep the product different from competitors.
"A differentiation strategy is one in which the product OFFERING is differentiated from the competitors." = the focus is on the strategy in the beginning, at the initial launch of the product. The "offering" is that moment when the product is first offered.