The first is the water. Just like Seattle is usually colder than Spokane during warm pattens because we're closer to the cool Pacific Ocean, the Puget Sound acts like a bit of air conditioning to Seattle and the west side that sometimes doesn't make it to the Eastside. (Yes, there's Lake Washington, but it's smaller and not quite as effective at cooling as Puget Sound).
Second, many of our warmer-than-normal days involve warm air blowing west down the Cascades. Thus, spots closer to the mountains tend to be a bit toastier.