Whomever told you that all i5 chips are quad core on a PC do not know what they are talking about. The fact is, every current gen Intel i5 mobile chip is dual core - whether they're in a Mac, Dell, HP or any other brand you can name.
Just head to Dell and check it yourself - take a look at their 15" XPS notebook line and you'll find the following i5 chips available:
i5-2410M
i5-2520M
Both dual core, because Intel is not making a quad core mobile i5 chip at this time.
And the mobile chips are used in practically every notebook made except your higher end gaming rigs because - they require less power, producing less heat and are easier to cool in the compact form of a notebook and thus provide longer battery life.
Here is a full comparison listing of the current mobile i5 chips -
link.