Hi, a similar question was asked recently, about the distributing version: why so few companies held compared to underlying index.
See link, but ignore comments about it being a synthetic, as you will see in subsequent comments that was incorrect.
Why so few companies
Basically, the ETF attempts to track the underlying index using a smaller representative sample of companies, to reduce cost. So it shouldn’t be markedly less diversified. The risk is that its performance is less aligned to the underlying index than a fund with more constituents (known as tracking error).