Sometimes one finds a reference to section 8zb of a law, and proceeds to refer to the law on legislation.gov.uk, only to realise that there is apparently no section 8zb there. One then has to google section 8zb uksi YYYY/NNNN, and then if one is lucky, they may find the law that effects section 8zb's insertion to the first law. Others, section 8zb is right there next to section 8b, and one is able to see the law as it was before the modifications, and with them integrated, and the date and cross referenced origin, etc, of each separate modification. What processes determine when each of these scenarios is the case?
Assuming the question is referring to legislation.gov.uk, which is the official home of UK legislation:
The website has a policy of updating primary legislation (i.e. Acts of Parliament, etc) as they are amended, but generally not updating secondary legislation (i.e. statutory instruments, etc), though it appears that policy may be changing.
In addition, updating legislation on the website takes time, so even Acts don't always appear in their latest form. However, when this is the case, there is normally a message at the top to indicate this.
(I assume that the reason for not updating secondary legislation is simply because of quantity. Thousands of statutory instruments are issued each year, so the National Archives (who run the site) may simply not have the staff to handle them in addition to primary legislation. Note that there are resources - e.g. Westlaw - who do do this, but their service requires a subscription.)