2 of my close friends have birthdays days apart. This year the 3 of us met up. I gave them both a present, expecting them to open gifts at home on their birthdays (as they usually do) but instead they opened them over lunch

Let's call them Anna and Sadie. I gave Anna perfume/skincare worth £50 and Sadie a book and chocolates worth £10. Anna spends a similar amount on my birthday gifts, while Sadie spends £5-10 on me.
Sadie was staying with me (she lives further away so I see her less often). She was moody that eve, I asked what was up, she said she was upset that I'd given Anna a nicer gift
I explained I spent the same amount as she spends at my birthday, also reminded her I'm on SMP. She was still upset that I'd 'treated' Anna to something nicer and felt I don't value her the same way.
If it's relevant we've all been friends for 15years, since high school. Sadie is a mature student, Anna has a reasonably well paid job.