A page is a complex item, requiring parsing of the page contents and then requests to multiple servers/sources. So, your page load time will be made up request time for an individual component, processing time for the page parsing and javascript (depending upon virtual user type), requests for the page components, where they are served from, etc... Compare this to your Wily monitoring, which may only be on one of the tiers involved.
For instance, you may have static components being served from a CDN which has zero visibility in your Wily Model. You might also be looking at your app server when the majority of the time is spent serving static components off of a web server, which is oft ignored from a monitoring perspective. Your page could have third party components which are loading which get counted in the Loadrunner time, but do not get counted in the Wily time.
It all comes down a a question of sampling. It is very common for what you see in your deep diag tool to be a piece of the total page load, or an individual request which makes up a page where there are many more components to be loaded. If you want and even more interesting look then enable the w3c time-taken field in your web HTTP request logs and look to see the cost of every individual request. You can do this in the web layer of your app servers as well. Wily will then provide internal breakdown for those items which are "slow."