Apparently, PayPal's cart system seems to be taking the last URL visited (referrer URL) before the cart page itself, and then making that the URL of their Continue Shopping button. This is a recent change to the way PayPal's cart behaves, which is causing all the problems reported with the "PayPal Cart" version of E-junkie's button codes lately.
If you're using PayPal's own cart buttons (without involving E-junkie), then all of the product specs are configured in the button code on your own page, so the buyer goes directly from your page into PayPal's cart when they click a purchase button, and thus your page would be the "last URL" that their Continue Shopping button links back to.
If you're using E-junkie's "PayPal Cart" version of button codes, then our button code only contains a reference to the product record stored in our database, so clicking the button invokes a script on our server that looks up the product specs and then translates those into a format that PayPal's cart system can recognize, and then redirects the buyer along with those product specs into PayPal's cart. Since the buyer is not going directly from your page to PayPal's cart -- they are actually going from your page, to our script first, and then into PayPal's cart -- the last-visited URL that PayPal's cart can see is not your page, but our script's URL. Thus, clicking PayPal's Continue Shopping button takes the buyer back to the last URL visited, which in this case is actually our script URL, which simply forwards the buyer and product data back into PayPal's cart.
Again, I will stress here that you do NOT need to use our "PayPal Cart" button codes merely to accept payments via PayPal. The standard E-junkie cart and button codes will show buyers a PayPal checkout option in their E-junkie cart, and this also provides full support for our other cart-based features.