The sliding scale shows the percentage of your income which must be paid back because of HECS/HELP. This is not a choice, it is a tax which must be paid.
The amount taken out of one pay is based on multiplying that fortnightly pay by 26.
Sometimes what happens is that your usual pay is just under a HRI level, but come the end of the financial year, after everything is added up, you are actually over that HRI level and therefore must make up the extra pay required by the higher HRI level.
It usually happens if you receive infrequent overtime or any other sort of extra pay.
This happens to me every year.
e.g.
Lets say your usual pay is $46 800 p/a or $1 800 p/f and you get no overtime.
Each fortnight you would not pay any HECS as your under the lowest HRI.
However one fortnight you receive $2 000 because of overtime.
Your tax for that fortnight would include a HECS/HELP repayment of $80.
($2 000 * 26 = $52 000, meaning 4% HRI repayment rate, $2 000 * 0.04 = $80)
Another fortnight you receive $2 200 because of overtime.
Your tax for that fortnight would include a HECS/HELP repayment of $99.
($2 200 * 26 = $57 200, meaning a 4.5% HRI repayment rate, $2 200 * .045 = $99)
You get no further overtime for the year.
At the end of the financial year your total pay is $1 800 * 24 + $2 000 + $2 200 = $47 400.
This puts you in the 4% HRI repayment rate category. The compulsory payment is therefore $47 400 * .04 = $1 896.
You have only paid a total of $80 + $99 = $179
Therefore you owe $1 896 - $179 = $1 717 in HECS/HELP repayments.
Hope this helps.