Soil pH meter - eh?

I bought one from B&Q, clearly a cheap generic one in a Verve packet. But I can't figure out what on earth (haha) it's on about. It gave fairly realistic readings at 6.5 except for one border which it said was 5 at one end. But also the clay subsoil it reckoned was also 6.5 as well as the compost filled beds. So I stuck it in a glass of tap water and it reckoned my tap water was 4.0. I'm pretty sure it's not. I added some baking powder and it went *down* to 3.5! And didn't fizz (obviously) so clearly the water isn't that acidic.

Is this broken or simply garbage? Or is there something I don't understand about soil testing?