Is the 8.4 the first amount of ARO you have earnt mining with us?
Yes.
I started mining recently and my pending amount increased to around 8.5 and then started decreasing whereas I was still mining
Ok, because you need to earn 10 ARO before a payment is made to your wallet, what will happen is that 7.7 will go from being "pending", get added to the 8.4 you have already earned and you should receive a payment of 16.1 ARO.
Any new pendings will then be paid as normal once you have achieved an amount of 10 ARO
Hope this makes sense

It makes sense!
I didn't know there could be "several" pending amounts.
(although it really looks like it's going backwards as in the last few minutes the pending amount went from 7.77 to 7.74).
Thanks for your answers, I will keep you posted if I receive any payment