Hi,
I do not use a Nikon camera, but it may be down to some setting that adjusts the picture ratio. I would think that on a full screen setting on the website, a 6x4 image would stretch further across the screen than a 7x5 (figures are inches - 2.54 centimetres/inch). The problem with changing such a setting may also adjust your megapixel figure, but that may not be an issue if you are not producing large hard-copy.
I'm no expert but the above may help, no doubt someone with a Nikon can help you further.
Best regards
Tom