The primary reason for this difference is because the RUM Aggregate Waterfall page actually pulls from a subset of data. Most of the pages in the RUM module (including Performance Details and Performance Overvew) will pull metrics from your site’s entire data set, whereas the Aggregate Waterfall page represents the data for only the sessions that collected WCD data. Depending on what your site’s WCD Collection Percentage is set to, that could be a substantial difference.
For example, if your site has a WCD Collection Percentage of 5% in the contract, then the Aggregate Waterfall page will show data for only that small percentage of sessions. By default, in this example the RUM Performance Details page could be showing you an onload averaged across 100,000 page views, whereas the Aggregate Waterfall page would show you an onload averaged across only 5,000 page views. Those two onload times will not match because they are pulling from different data sets.
However, you can change the filter options in the RUM Performance Detail / Performance Overview page to match the Aggregate Waterfall page. There is a filter option named “WCD Data Inclusion”, and you can set that to “WCD Data Only” (in addition to the rest of your filter options) to get the same performance metric data in both pages of the portal.