There are some old threads where this topic was discussed in detail. There are various credit characteristics that formulate Beacon score (Equifax) or Empirica score (Trans Union). FICO score is actually a US scoring model, but since both Canadian bureaus use Fair Isaac built scorecards, many credit people use the term FICO in Canada as well.
Now, many scoring models including FICO tend to separate Revolving debt (credit cards, etc.) vs. Installment Debt (car loans, etc.). When scoring revolving debt characteristics, particular attention is given to Utilization. The rationale is that a) if you choose to carry balances and pay min payments, statistic analysis shows you are a higher risk than somebody who pays balances in full; and b) it is your choice as a consumer how much of your credit limit to use up. Installment Loans tend to be given for the exact amount of the purchase, hence utilization is not as predictive.
Your LOC is normally treated as Revolving Debt, whereas Mortgage would be treated either as an Installment product, or specifically a Mortgage product. So when you highly utilize your LOC, the scoring model spots that you are not paying your revolving debt in full, and therefore, "punishes" you for it by assigning fewer scoring points. The higher is the utilization on your LOC, the worse the "punishment". Under 50% utilization is generally not triggering a lower score. 50-75% will cost you some points, whereas over 75% will definitely be adversely reflected in your score.
Hope this helps.
