Type I second degree AV block is also called Wenckebach block or Mobitz type I second degree AV block and is usually due to a conduction deficit in the AV node. There must be at least 2 conducted beats in a row before a beat is dropped to make this diagnosis. By definition the PR interval of the second conducted beat will be longer than the PR interval of the first conducted beat. If additional beats are conducted before a QRS complex is dropped, those beats will have longer and longer PR intervals until a beat is finally lost.
If two complexes are not conduced in a row and every other beat is dropped, this can only be called second degree AV block. This is because you only have 1 PR interval to look at and you need two PR intervals in a row to discern type I from type II second degree AV block.
See examples of type I second degree AV block