It doesn't really sound like there is too much to worry about. Since he is gaining weight, then he probably has grown. Like Bridge said, it depends on who is measuring and if they measure the same way every time.
My doctor will measure but doesn't put too much weight on the masurment, because you have to have that on the chart, but sometimes the nurse has measured differently than one previously, or the baby moves a bit or something, and sometimes my baby has SHRUNK!! lol
So don't you worry: weight gain means that the baby has used what he needed to grow and then put the weight on for later. Weight gain means growing. No need to worry, your munchin is fine