Using "- input" process in "Inputs"

I have a V2G charger which imports and exports electricity from the grid. I have created a feed to calculate my house usage by taking the power reading from the mains meter P1 (use) and subtracting the V2G power (P2). Here is the process list from my P1 (use) feed:

However I am getting some unexpected results. Here are some results from these feeds. As you can see on the first two lines correctly calculate the required “house” value (238 - 45 = 193). But the “house” value in line 3 should be -2389 - -2586 = 197
And the 3rd and 4th lines up also give incorrect values.

Have you any ideas why I am not getting the expected values please?

“Unix timestamp”, “emontx3cm15:use”, “emontx3cm15:V2G”, “emontx3cm15:house”
1597676400, 238, 45, 193
1597676410, 238, 46, 193
1597676420, -2389, -2586, -2435
1597676430, -3309, -3507, 199
1597676440, -3309, -3506, 198
1597676450, -3308, -3505, 198
1597676460, -3306, -3504, 199
1597676470, -3306, -3504, 198
1597676480, -3306, -3503, 198
1597676490, -3305, -3503, 198
1597676500, -3305, -3502, 198
1597676510, -3305, -3502, 197
1597676520, -3305, -3501, 197
1597676530, -3307, -3503, 194
1597676540, -3304, -3501, 199
1597676550, -923, -1116, 2578
1597676560, 236, 46, 1352
1597676570, 236, 46, 190
1597676580, 236, 46, 190

I’m not an emonCMS expert, but it looks like a timing issue. The general advice is it’s preferable to use “+ feed” rather than “+ input”, which I’m told removes the problem.