My sister went to the Westcliff 11+ exam and got quite a hard paper. That would be fine, however when she asked her friends who went to the session after, they got a different paper, one significantly easier. The scores of hers and her friends reflect this greatly. My question is, do they consider people differently based on whether or not they took the easy test/hard test

In what ways was the paper 'harder'? All you have to go on is the personal opinion of a single person. It is in the interests of all parties for the level of difficulty to be the same among all papers, so I suspect it's more to do with your dsis than the paper itself.

The ones who were in the first session and the ones in the second session discussed what was in the papers and the second session people all spoke about how it was extremely simple

If you know the answers, it’s simple, if you don’t, it’s hard. It would be very unusual for a school to accept 2 papers that were completely different in levels, so I suspect it is more to do with “perception and ability” than actuality.

Were these sessions straight after each other?

Exam papers in different sessions on the same day tend to be the same paper.

I have not heard before of papers being different on the same day.

DG

Wasn't on the same day I don't think. Will check with her

but doesn't making two papers create that risk?

I know nothing about the area but this page https://www.elevenplusexams.co.uk/schoo ... ex-11-plus suggests that there is only one date available to sit the exam. Previous years' papers are available so it would be obvious if there were two papers used in the same year. Of course it is possible that a second paper is used for late takers who may have been ill on the first date, I don't know. Presumably that wouldn't be several of her friends though.
Some people find some exams easy, others don't. That's how there are a range of scores for any exams.

If it's WHSG, it would have been the CSSE main sitting on Saturday September 23rd and the reserve date sitting on Tuesday October 3rd (assuming that we are talking about 2017?). There are always two papers - they are written in-house, they've been doing it for over twenty years and I'm pretty sure that every year there are people saying, if only my DC had sat the first paper / sat the second paper, they would have done better. I can't see that the CSSE would suddenly come up with two English papers of completely different objective levels of difficulty after all these years.

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The backup date is for students who were ill on the day.

Surely your sister can't have more than one friend who was ill on the day and had to attend the backup date? DG