Chris,
Please check out Q1 and Q2 on this page...
Basically Sage (unlike all other payroll software that we're aware of) duplicates HMRCs paper NI tables, whereas everyone else (including HMRC themselves on their website) uses an HMRC specification called the "exact percentage method". So you will see small differences in NI, up to a few p/week or /month. I wouldn't worry about them.
You should always get a perfect match on PAYE though.