Float label combobox default height

Hi,

If I use a responsive form and add some of the bootstrap float label components (floatlabeltextbox, floatlabelcombobox) they have different default heights. The textbox is 40px and the combobox is 45px. This makes aligning the fields quite difficult where you have flex containers with separate columns. I cannot seem to find where the difference in heights is coming from. I have used the chrome developer console to inspect the CSS but cannot identify the class or classes that are controlling this.

[attachment=0]bootstrap.png[/attachment]
I can fix my problem by adding a class to my own CSS but would like to understand the root cause of it.

Any help would be appreciated.

Thanks
Steve