Yeah, the base value is basically a cushion that either adds or subtracts stability, so that it always falls within 0-100 and on its own provides 40 stability as a freebie.

That way gains and losses math out to 0 if the Stability would be below 0 and 100 if it would go above 100, in those cases you need to look at base stability to see how much correction is applied and learn how good/bad situation really is (look how much it is below/above 40, in this case you need 407 just to get to zero). It's one of those things that become intuitive once you know what to look for, but seem like they overcomplicated things because game isn't good at signalling what's going on.

The math of it is correct, you should only read the left column, the numbers slightly to the right are not additional stability, but a breakdown of value above, example from the screenshot: +20 from Aqueduct and +15 from Public Fountain sums up to +35 from Infrastructure.