Two controls in Koda realization are container controls: Tab and Group (speaking strict, form itself is container too). This means that any other control placed onto them will have it as parent. This result into some features:
sub controls will move together with container control;
position of sub controls is relative to container control;
if you want to place controls that were created outside a container control onto it, you have to copy/cut those controls and paste them onto desired container control. Drag'n'drop will not work in this particular case.