That's (somewhat) true for accounts that represent stocks (assets, liabilities, not really for equity though), it's not true for accounts that represent flows (income, expenses). Income is recorded as a credit entry in an income account, eg (the corresponding debit entry would typically be on something like a current account or claims on customers).
Consider the positioning of income and expense accounts within the accounting equation. Essentially, they are components of equity. View equity as the company's obligation to its shareholders. A credit entry signifies an increase in what the company owes, whether to creditors or shareholders, while a debit entry reflects an increase in the company's assets.
