1 unfortunate girl got an E in Maths, which means "only" 99% of the year got the 5A-Cs plus English and Maths that is used to rank most of the tables published in the media, which means NW gets listed beneath any schools that got 100%.
In terms of average point scores (which is the second, tie-breaking criteria) it looks to me that NW is something like second in the country (at least among the Top 200 schools that are included in the Guardian's data). So that's where it would be if it wasn't for that stray E. I'd argue this average points score is much more important when you're comparing these sort of selective schools than whether a school gets 99% or 100%, which is mostly determined by whether a few kids have an off day or not.
That being said, NW's average point score (796) in the DfE data seems inconsisent with the data the school reported at GCSE results time. (685) I don't know why there is that discrepancy. That would push the school down to something like 15th in the country. This is still significantly ahead of all of the comparable schools in the London area. It's quite an achievement IMO. That's an awful lot of A*s, I don't know how they fit it all in.
I would still take these tables with a pinch of salt. Schools that do iGCSEs don't get listed at all. Private schools tend to do fewer exams. It's hard to compare like with like. I don't think that enormous points score means the school is better than a school with a lower score. Approach this data with caution.
But as a fun/silly exercise for those of us applying for schools this year, I thought I'd correct the impression that NW didn't do well in the tables this year: I'd argue that it's scored a remarkable success! I'm delighted!