On the other hand, bold is implemented as being bold text so you know that if the device or browser supports it, your text will be bold. With strong you don't know how the browser could have implemented it, so you need to exactly set the stylesheet information to match bold. Some kind of exotic device could display strong elements as italic or underlined, so you need to actively set in the CSS you don't want that. Furthermore you don't know what else of weird things the device may do to the strong element.

I'm not saying that bold is better than strong in semantically correct HTML, I'm just saying you're more certain of what will happen with your layout and design if you use bold instead of strong.