When i save my psd file into jpeg file using photoshop, the resolution of my jpeg is different from psd, why?

when i save my psd file into jpeg file using photoshop cs5 , the resolution and size of my jpeg file is different from my psd file(resources)..what is happening?
Update: here's the situation of my problem, i made some design using photoshop cs5, the image size that i made is (width : 60cm, height : 40cm, resolutions : 300 pixel).
and when i saved into jpeg file the image size changed into (widht : 76cm, height : 50cm, resolutions :240 pixel), why???
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