As you point out,
Higher numbers of reviews compared to others will inevitably skew the
figures.
This phenomenon is not restricted to package delivery but anything that
relies on opinion/experience Vs expectation of service rather than solid
hard delivery figures.

You can see a logical pattern in those trust pilot scores you posted.

DHL, PF and RM are the heavyweights. They have better working conditions,
better paid staff and overall provide a 'consistently' better service. The
caveat being a more costly service with a much higher customer expectation.
Their on-time deliveries figures are publicly available and recently RM
were heavily fined by Ofcom because their 1st class service failed to meet
its 'on-time' target of 93%. (I can't even find publicly available figures
for any of Evri's delivery targets... But Imho doubt it is anywhere near
the same)

Yodel, DPD, Evri are all (for want of a better choice of word) 'budget'
couriers.
As a result the expectation of service is generally lower and there Ofcom
targets will be significantly lower, therefore much easier to achieve and
even over achieve!
The caveat here being lower paid staff with higher pressure put on them
resulting in huge staff turnover and more issues with delivery.

Like another person posted, reviews get posted mostly for bad service. But
how often do you post a good review because your 1st class RM birthday card
arrived on time, or even a day early. You don't because it's expected!

Evri on the other hand has such a bad rep that if it arrives at all people
are likely to post a good review!
It was a few months ago now and I can't find the actual review or i'd post
the link for reference.
It was an Evri parcel delivery review, it was something like 7 days late
and had been left in a questionable location (possibly a full wheelie bin,
but can't recall) it was was a 5 star review because their expectation of
the courier was soo low based on their previous experience.