Shares of J.C. Penney (OTC:JCPN.Q) and Best Buy (NYSE:BBY) were big winners last week, soaring 29% and 8%, respectively, after the retailers posted better than expected results for the holiday quarter. A big part of the market's excitement was a spike in online sales at both companies, but there's more to drumming up dot-com buzz than meets the eye.

Let's start with J.C. Penney, where sales at improved 26.3% to $381 million during the telltale period. That's great, but it's still just 10% of total sales. Is that enough to move the needle? Yes, but not in a very flattering way. See, J.C. Penney is one of the many retailers that tack on Internet sales to their comparable-store sales. The department store chain takes that $381 million and allocates those sales evenly across its individual brick-and-mortar locations.

That's a pretty big deal in this particular case because J.C. Penney reported that comps rose 2% during the fiscal fourth quarter. There was a lot of hoopla surrounding the struggling retailer's store-level performance turning the corner, but did it? You don't need to be a math wiz to figure out that if 10% of your business rose 26% and your comps rose 2% it means that the remaining balance -- the 90% of the revenue that was actually sold at physical stores -- was slightly negative.

Unlike J.C. Penney, Best Buy could not post positive comps over the holidays. The consumer electronics superstore operator saw same-store sales dip 1.2% during the quarter. However, aggressive online pricing and marketing strategies helped it come through with a 25.8% spike in Internet sales, accounting for 11% of Best Buy's revenue. Yes, Best Buy also bakes its sales into its comps, so we're looking at physical same-store sales actually dropping closer to 4% if we eliminate the dot-com component.

Don't get me wrong. It's great that J.C. Penney and Best Buy have been able to post strong growth through increased online efforts. It proves that there's still some appeal to the brands. However, investors assuming that J.C. Penney stores were ringing up more sales than last year or that Best Buy's store-level activity only slipped 1.2% are missing the footnote at the end of the reports. 

J.C. Penney and Best Buy have yet to bottom out.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.