I will have to go with @michal-szulc on this one. However I do think you have a valid point as well @Chrono79.

The charcode for A is 65, and it needs to map to 1 in this exercise. The rest of the letter charcodes are followed alphabetically in ASCII/UTF-8

can someone explain the '-64'please.

Errr, no. Sample tests is a section where you can write your own tests, normally a few tests are shown there by default and you can add your own if needed. Attempt has (also normally) the tests of sample tests plus more tests, some of them with input randomly generated so you can't hardcode your answers. You test your code against the tests in sample tests with Test button, and the full test suite (again, sample tests aren't tested here unless they're also written in the full test suite) with Attempt button. If you pass the full test suite, that button changes to Submit and you can submit the final version of your code.

Attempt button takes test data from "sample test" puts test data into your code and compare it with expected results.
Test Data and expected results are in "Sample Test".
If your code provides all results as expected results then you would see "Submitt" button. When you push it your code will go through series of tests, heavy tests which would verify lots of other random input and bad input, critical input etc. Full series must be finished within 16 seconds. If your code would be to slow you will get an error message "Time out"

best regards
Michal

can someone pls explain how the 'attempt' section works?