score:4
There are several possibilities: either your wife has deliberately been granted 6 months instead of 2 years, or your wife has mistakenly been granted 6 months instead of 2 years, or your wife has been granted 2 years but the wrong vignette has been applied.
It is possible for a shorter duration to be granted than was applied for:
You can issue a shorter duration visa if the applicant meets the Visitor rules relevant to their visit, but you have concerns about issuing a long-term visit visa. Entry Clearance officers must obtain the authority of the Entry Clearance Manager before issuing a visit visa of shorter duration than that applied for.
You must provide clear reasons for the decision to grant a visa for a shorter period than that applied for in a covering letter when the passport is returned.
In such cases no refund (full or partial) is available.
Whatever has happened, it would be clear from the decision letter, which it seems you haven't been issued. In the first instance I would get in touch with your VAC, and ask if they wrong vignette has been applied, and mention you haven't got a decision letter. If the wrong vignette has been applied then they should be able to correct this.
Additionally, I would recommend you immediately make a 'specific' subject access request to the Home Office for the decision letter.
If the smaller grant of validity has been given, either deliberately or mistakenly, then there won't be a way to correct this short of applying for a new visa, and there is no right of appeal, nor way to get a refund. If a mistake has been made you might wish to make a complaint.