**Why are some _Doctor Who_ regenerations the same in classic Who?**

I know in "new" Who regenerations are supposed to be the same but why are the regenerations in classic Who not all different?

For example:

 - The Second Doctor's regeneration, 2 regenerates to 3, is the same as the Fifth Doctor's regeneration, 5 regenerates to 6. They both have swirly faces around them.

 - The First Doctor's regeneration, 1 regenerates to 2, is the same as the Third Doctor's regeneration, 3 regenerates to 4. They both just morph faces.

Yet every other _Doctor Who_ of (classic) Who is different, **is there a reason why these particular Doctors have similar regenerations in classic Who whether in universe or out of universe**?

Did the same producers from the Second Doctor era suddenly help with the Fifth Doctor's regeneration or is there another reason that the Doctors mentioned above have so similar regenerations?