Holls indulge me here, but I've just looked up your son's figures on this thread. From my reckoning:
On 13th you posted your DS was 16lb - so 16weeks
On 10th you posted that his previous weight was 15lb 5 (I assume weigh in would have been the previous Tuesday on 6th) - so 15weeks
Today you've posted he was 15lb 15 - so 17weeks
That measurement on the 6th would put you about 3/4 of the way between the 50th and the 75th centile.
That measurement on the 13th would put you almost bang on the 75th centile
That measurement today would put you about 3/4 of the way between the 50th and 75th centile
Question: If you hadn't weighed in last week, would you be worried today?
Therein lines the problem with weighing in every week. We are talking a difference of 280grams here. The same as 280mls.
Your HV is right. Its the difference between a poo and a feed and if you look at this over two weeks, your son hasn't fallen below the curve anymore. It looks like your son grew one week more than he 'needed' to, and 'less' than he needed to the following week. But overall it balanced out.
Hope that makes you feel a bit better.