It's an honest mistake. Also from The Christmas Carol, Chapter 3, where Scrooge views the Cratchit's Christmas dinner with the Ghost of Christmas Present:

At last the dishes were set on, and grace was said. It was succeeded by a breathless pause, as Mrs Cratchit, looking slowly all along the carving-knife, prepared to plunge it in the breast; but when she did, and when the long expected gush of stuffing issued forth, one murmur of delight arose all round the board, and even Tiny Tim, excited by the two young Cratchits, beat on the table with the handle of his knife, and feebly cried Hurrah!

There never was such a goose. Bob said he didn't believe there ever was such a goose cooked. Its tenderness and flavour, size and cheapness, were the themes of universal admiration. Eked out by apple-sauce and mashed potatoes, it was a sufficient dinner for the whole family; indeed, as Mrs Cratchit said with great delight (surveying one small atom of a bone upon the dish), they hadn't ate it all at last! Yet every one had had enough, and the youngest Cratchits in particular, were steeped in sage and onion to the eyebrows!

But here's the thing: the Turkey was an EXOTIC bird in England. Thus, Scoorge, to make amends, bought nothing but the best.

Here's a good weblink on this switch of the birds:

http://realbook.com/news/dickens-christmas-carol-and-question-turkey-or-goose