Billing is calculated based on per-minute memory usage as shown in the per-service Metrics tab, not the Observability dashboard. The Observability dashboard widgets and the per-service Metrics tab can show different values because they may cover different time windows or use different aggregation - the first screenshot shows a full day view with both services overlapping, while the individual service views show a shorter window. Your current billing period shows $3.16 in memory for "Github: realestate" and $2.54 for "Postgres," consistent with the per-service metrics (~200MB and ~100MB respectively running continuously). You can always check your exact billed usage at the Usage page.

all three screenshots are based on 6 hour window. observabily graph never goes above 100mb, while ithub service is steady under 200mb.

Well in your observability graph, the purple line represents the memory your Postgres is using, not both (There's a legend at the bottom of the graph).