I suspect that your issue is not directly related to the mapping of the express lanes. I think the problem is simply that there are many routes from south Arlington to Ashburn that differ by less than 5 miles that waze can choose from, but waze only gives the 3 alternate routes. If waze's data doesn't match your experience, your route may be left off the list of options.
Regional Coordinator: Mid-Atlantic, US
Verizon, Nexus 6, Android 6.0.1, Waze 220.127.116.112