The router will use some form of hashing to 'pin' connections to each route, for example source and destination IP hash. Think about it from an IP perspective - the firewall in front of the target will be NAT'ing also based on (it's view of) source IP. A single TCP connection cannot span two source IPs so traffic randomly arriving from a different IP (i.e. the second connection) would simply be dropped.

With the speedtest, I would guess that it is using a bunch of TCP connections probably to a few targets that run in parallel to attempt to max out the line as much as possible, hence why you see the aggregate.