How calculate the Sum of 2 network interfaces (error in the sum calculation)

Grafana
1

Hello, I would like to calculate the sum of 2 network interfaces in 2 servers.
I created a simple graph with 2 queries

Query A
SELECT non_negative_derivative(mean(bytes_sent),1s)*8 as “out1” FROM “net” WHERE host =~ /thefirst/ AND interface =~ /eth0/ AND $timeFilter GROUP BY time($interval)

Query B
SELECT non_negative_derivative(mean(bytes_sent),1s)*8 as “out2” FROM “net” WHERE host =~ /thesecond/ AND interface =~ /eth0/ AND $timeFilter GROUP BY time($interval)

The graph of query A indicate 1GBit/s
The graph of query B indicate 0,7GBit/s

But, in the query C, I would like to add the 2 queries.
SELECT non_negative_derivative(mean(bytes_sent),1s)*8 as “out_total” FROM “net” WHERE host =~ /thefirst|thesecond/ AND interface =~ /eth0/ AND $timeFilter GROUP BY time($interval)

The result is 6,4Pbps… Could someone understand why the result is not 1,7GBit/s

Thanks.