It simply does not add up

Incident-1: One of the all under one roof home furnishing stores.

The conversation goes something like this

I like this chair, I will take it , how much is it ?
940 sir
Here you are, I have a Rs.1000 note
Sir can you pay me 60 more ? I can then return 100 to you 
Errr …. Why do you say you want 60 ? Are you planning to pay me back 120 ?
No, sir I will give you back 100. See the chair is 940, you paid me 1000, so if you give me 60 more, i can give you 100
If I give you 40, you need to pay me back 100. If I give you 60, you need to give me back 120 !!

Total confusion, 2-3 minutes of patient explaining on how addition works, finally she gets my point. Well it is late in the evening and she must be getting tired, I think

Incident-2: A famous retail store

How much is the total bill ?
309 Sir
Here is 1000
Do you have 9 Rs change ?
Nope, but here is a 10
Here is your balance, 601
601 ??? Haven’t you missed a hundred ????
No sir it is correct. The bill is for 309, I gave you 601
And if you add the two what will you get ?

System crash. Cashier just froze. She calls her manager to figure out this tough math problem. Manager thankfully is an older guy who can still add 2 numbers in his head, so I get my 100 without too much hassle.
Surely this must be a coincidence, I think. Our math skills have not gone so bad, have they ?

 Incident-3: Yet another famous retail store

Your bill is 240 sir
Ok cool, take this 500 note
Your change sir, 160
What the …., aren’t you making a mistake, I should get back 260 !!

Again 2 minutes of confusion, but finally I get my additional 100. Man , surely something is stinking, and this time it has got nothing to do with the food in my refrigerator !!
------------------------------------------------------------------Common trend between all three stores
(a) The cashiers did not enter the amount I paid to their billing machine and did the math on their own.
(b) All cashiers are kids in the 18-21 age group

Question I have is this. Has the proliferation of Cell-phone calculators and computers affected our basic math skills so much ??? 

One thing is for sure, gone are days when I used to take my change and blindly shove into the purse without counting !! I am counting every note from now on !!