Pending and reserved amounts

i want clarity really because i feel ********** sometimes, i need to understand the cheque process how the pending and the reserved work.

so i have a cheque account, i swipe for something, it shows me the money is reserved with that amount but then the amount is taken fro my available amount balance and thrown into the balance. and this happens everytime i do something with my card, then when the transaction goes from pending to actually being paid, the balance decreases with that aount.

so in my understanding, the balance amount is like a container carrying all these transaction that are pending everytime i swipe.

what i do not get is why sometimes transactions that have already decreased my amount in available balance would come and debit again when the amount should be in the balance account container?

shouldnt my balance and available balance total the amount i get, lets say the salary that was paid in?