Why is the numerator adjusted for convertible bonds when calculating diluted EPS?

Convertible bons are a unique combination of debt and equity, they provide investors with the chance to convert a debt instrument into shares of the issuer's common stock, at a set price and usually by a set date this is why they have this dilutive effect as  are not common stock in form, but allow the owner to obtain common stock upon exercise of an option or a conversion privilege. Of course in the numerator we will find the equity part which is measured according to the terms of indentures.

If the company has convertible preferred stock, use the if-converted method:

Method assumes that convertible preferred stock have been converted to common stock, there is no need to pay the preferred dividend. Thus, the numerator of the EPS ratio increases because the preferred dividend need not be subtracted hence while calculating diluted EPS numerator adjusted for convertible bonds.