That would be scientific notation and it is all on excel's end of the world. And it is pretty tricky to get around if you have to pass in flat files, such as CSV, as there is no way to indicate that it should show this column as a number with format X. To get past it, you'll need some better ways of generating excel. I recommend either CarlosAG's excel xml writer or using the excellent excelpackage library.
That is a function of the way excel formats data without any input -- which is what happens when you have