I am pretty sure they are just different intervals.
Iphone prices will be first/ website second (prices may be different depending on your country.
20 credits: 0.99/ Can't buy
50 credits: Can't buy/ 2.90
120 credits: Can't Buy/ 6.90
250 credits: Can't Buy/ 12.90
450 credits: 19.99/19.90
1000 credits: 44.99/43.90
So until 220 credits it is better to buy credits (by 20 only) on iPhone.
Seeing how 120 credits is only 5.94 on iPhone. You save close to a dollar. However at the next increment it is more worth it to buy on the computer.
Depends if you have an iphone if you don't then use the computer. If you do then i don't know because I don't have an iphone
You are correct. It is worth getting credits on the iPhone by increments of 20 until 220. After that it's easier to get on the computer.