2G/2G FDX on XB10: upload 2.4 Gbps but download capped ~1.55 Gbps, only one downstream OFDM channel bonded

On a 2 Gig symmetrical plan with an XB10 in bridge mode. Wired tests from a 10 Gb/s server (Ookla + 16-stream Cloudflare) consistently show ~1.55 Gbps down / ~2.35–2.4 Gbps up. The router's own WAN test matches (1.45 down / 2.41 up), so it isn't my LAN.

Gateway status: downstream bonds only 5 SC-QAM channels (24–28, 789–819 MHz) plus one OFDM channel (193, 900 MHz). No second OFDM channel. SNR is 47–49 dB on all, power 7–10 dBmV, all locked. Upstream bonds 4 ATDMA plus 4 OFDMA channels (43/50/51/52, 36–297 MHz) at 28–37 dBmV, which explains the full upload speed.

One OFDM channel plus 5 SC-QAM channels works out to about 1.6 Gbps, which matches the cap. Is a second downstream OFDM channel provisioned on my node, and if so why isn't the gateway bonding it? Happy to DM account details.