I have discovered a deceased person in a modern electoral before. Errors happen, and in your case if you think other evidence points to it being the same person, it is probably the same person.
According to The completeness and accuracy of electoral registers in Great Britain (March 2010), the number of deceased individuals in electoral registers is (in 2010) negligible. Among the data given for several localities, the only place with a non-zero percentage of deceased electors was Derby with 0.2% of the electors on the register actually being deceased (see p 78).
On page 110 of the same document:
Since Electoral Registration Officers (EROs) update the registers each
month using updates of recorded deaths from the local registrar,
redundant entries relating to deceased voters are minimal. As a
result, the cause of the great majority of redundant entries will be a
- those electors who have moved out of or within the local authority after the annual canvass but have not notified the relevent ERO that
they have done so
- those electors who have moved since the last annual canvass, and whom an ERO opts to 'carry forward' from the previous register in the
absence of a response to the annual canvas.
As more and more systems have become computerized, I think the number of these types of errors has probably decreased. In 2002, it's possible that there were more errors of this kind made than in the 2010 report. When a person dies in England today, numerous agencies are notified with a single click.
More information about electoral registration can be found on The Electoral Commission website.