Multipliers explain most of it
Many providers bill premium routes at a multiplier, so 1 GB of real traffic is deducted as two or three. That's what x2 or 1.5x in a node name means. Switch to a node marked 1x and compare again.
The two sides count different things
- The client counts application-layer bytes; the server counts what crosses its interface, including TCP retransmissions and protocol headers. The second number is naturally larger.
- Uploads count too. Cloud-drive sync and seeding torrents run in the background and quietly eat gigabytes.
Check it yourself
Stop every node but one, download a file of known size — a 1 GB system image is convenient — and compare how much each side went up. If they roughly agree, it's a multiplier or a background task. If they're wildly apart, then go to support.