That's quite a substantial decrease.
There might be several reasons for that:
- the blocklists you are using contain less relevant domains
- you have allowed your browsers to use alternative DNS servers to 'help resolve navigation errors'
- IPv6 has become more prevalent (i.e. you upgrading to Win10 and using more smartphones, tablets, etc.), but your IPv6 configuration does not comprise (or even allow comprising) Pi-hole
- you have opted in to use a DoH (DNS over HTTPS) experimental feature in your browser (once this moves from experimental to mainstream, it has the potential to affect all DNS-based filtering )
- your surfing behaviour changed to visiting sites less susceptible to blocking
l've listed them in no particular order, and the list is probably far from being sufficiently comprehensive.