The 30-06 is based on the 30-03 that replaced the 30-40. The case of the the 30-03 was shortened to make the 30-06. the 270 and the 280 were both made from the original 30-03 case, they have a longer case length. The 25-06 was based on the 06 and share the same shortened case. So in reality they were all based on the 30-03, even those with much shorter case lengths.
The 30-.03 was just a longer necked 30.06. The 30-06 fires safely through 30-03 chambers.