Could someone explain dilution like I'm 5? I haven't researched it much, but I always assumed additional shares would be split between existing shareholders.

For example, let's say there are 10 shares in a company, and I own 1. We decide we need 100 shares in total. Dilution makes me think 90 new shares are issued, and I still own 1, bringing my ownership down from 10% to 1%. However, common sense tells me if you issue 90 new shares, since I already own 10%, that means 10% of the new shares should belong to me. So, I now have 1 + 9, or 10/100 shares, remaining at 10% ownership.

Why is the above not always the case?