Some points
A great-grandfather of mine was described as both a gardener and steward, so I see no conflict between the two descriptions.
The 1920 document was indeed required for pension purposes. As your g-grandmother was seeking to obtain an old-age pension, and needed documentary evidence from the census to substantiate this, I would tend to think that the parental details given on this form are correct.
Which then brings up the following point. Why the very, very different details on the second marriage? Different for both parents. For one parent I can well believe in a simple mistake - I have seen mistakes on multiple marriage certs, But mistakes for both parents? The thought that leaps to my mind is bigamy. Do you know if the first husband was dead?