I think I'd be inclined to query that with the Record Office. The description of the records includes this:
The records, both administrative, committee minutes and medical case records, are a well-kept and complete series [my emphasis]. Unfortunately the early signed minutes of the committee (c.1857 - 1883) are now missing. Some of the case books were damaged by flooding, but most are in reasonable condition.
This can be found at
http://www.rootschat.com/links/01m4i/ near the top of the page; there are also useful notes under Medical Records part way down the page. This page refers to the first deposit of records, and I couldn't see anything similar about the later deposits, but it would be strange if they hadn't been kept on a similar basis.
There are well over 500 items relating to the hospital under reference H/Wi, including large numbers of case records, admission and discharge books etc. On the face of it, it looks as though it could be a fairly complete set, and I didn't see any mention of it being only a 2% sample. Or, supposing say 200 of these references relate to medical-type record books, are they saying that there were originally 10,000 of them?