Answer:
To have a high credit score, you must have been consistent with your debt repayment since you started taking loans.
Your friend would therefore be incorrect in assuming that all those who did not miss a payment in one single year will all high credit scores because your friend is not taking into account the previous years.
Some of those people might have delayed or defaulted on previous loans for one reason or the other but were able to keep up with new payments in 2015. Whilst this would improve their credit score, it would still be low as a result of the actions of the previous years that decreased it.
