perplexing pixel perceptions....?!...

I've just ranted on about the sizes of images and pixels and now I find myself a bit perplexed by the fact that windows preview (and the software that I used to edit the original photos together on) seem to be showing a different rate of 'dots per inch' resolution then my browser....am I missing something here?

this is a screen capture of my browser at about 500 % zoom (approx.) on the left and windows previewer zoomed in to match the size of the browsers size (as a point of fixed reference for the resolution) as an insert image on the right...I'm not surprised that windows is in some way inferior, I just don't understand why!

any tech-heads out there know the answer? (open the image by right clicking on it so it opens in a new window and the disparity will be even more obvious!)