Thinking Sphinx - shows false positives even though delta indexing is used

Question

This may have to do with Sphinx rather than Thinking Sphinx.

I am using delta indexes.

1. Lets say a record has text field contains word 'foo'
2. Full indexing is done
3. word 'foo' is removed from the record
4. delta indexing is done
5. Search for 'foo'
6. The above record is returned even though it doesn't contain word 'foo'
7. Expected Result: Above record shouldn't be returned.

I am not sure if this is configuration issue to tell sphinx to ignore records from main index if the same record is present in delta index.

UPDATE: Can some one please respond to this?