Yes this is normal.
If you have multiple credit cards / LOC with multiple banks, and you use it on a regular basis, they will just keep increasing it.
Your income really has nothing to do with it. Your initial credit limits are determined by your credit report, and subsequent increases are determined largely based on your history with the bank.
I have credit with RBC / CT / AMEX. RBC has no problem dishing out 20k+ on a credit card, neither does AMEX. I'm sure I could get a ULOC with RBC as well if I wanted.
The weird thing is, banks don't talk to each other, and your limits are largely based on your history with the bank, not your credit report. So it might take bank A a while to extend you 50k in unsecured credit... But they will eventually, just based on your history with them. Meanwhile, you could be slowly accumulating this same type of credit with bank B, bank C, etc...
I'd imagine if you really wanted, you could accumulate 200-300k in unsecured credit, even without making much money, just based on having multiple credit products being used with multiple banks, and a long history with each bank.