sometimes a person wants a different color in their bobbin and all they have in the correct color is a different type of thread than the top, sometimes a person is just using up what ever is on their bobbins & not worrying about it, sometimes a machine 'behaves' better with the same in both areas, sometimes the machine seems to like the difference--there is no right or wrong- as long as your machine is ok with the threads you are using and you are happy with the threads you are using ---do what works for you. I often use silk thread on top and cotton thread in my bobbin. just me- my machine accepts this & I like the results I get- but it depends on what I am working on - every project is different and I make decisions based on the project- not based on what the lady down the street would do.