What you wrote is a relative relationship whereas acidity has an absolute relationship with proton concentration. So let's use a concrete example to help you see. Acetic acid is a weak acid - it's just varying degrees of vinegar. The formula is: CH3COOH. That carboxylic acid proton is the acidic one. When that dissociates, you get what? You get CH3COO- and H+ in a 1:1 ratio. That's why [H+] and [A-] are always equal - you can't get a proton to come off without also generating the acetate because the acetate is what's left after the acetic acid gives up a proton. Note that since acetic acid is a weak acid. So say you start with 1 mol of acetic acid and 5% of it dissociates. So now, you're left with 0.95 mol CH3COOH, 0.05 mol H+, and 0.05 mol CH3COO-. So only a few protons are free. Contrast that to a strong acid where almost complete dissociation occurs. So like HCl. If you start with 1 mol of HCl, you will end up with 1 mol of H+ and 1 mol of Cl-.
Two things here - remember that water is a very weak acid. There are usually only 1 x 10^-7 moles of H+ and -OH in 1 liter of water and that's what makes the pH of water 7. 1 liter of water is 1 L x (1000 mL/1 L) x (1 g/1 mL) x (1 mol/18 g) = 55.5 moles. So out of those 55.5 moles, only 10^-7 moles dissociated. That makes it a weak acid. Also note that if there are 10^-7 moles of protons in water, there is an equivalent amount of hydroxide because hydroxide is the conjugate base. But it's not this fact that makes water neutral, or pH = 7. It's the 10^-7 [H+] that makes it pH = 7.
I think you also read the question wrong - it says pH is 3.7, not 7.
