LR's matching criteria for keywords is badly designed and confusing. To troubleshoot this most effectively:
1. Do the menu command Metadata > Export Keywords and attach the exported "Lightroom Keywords.txt" file here. To attach a it here: After posting the reply, click More > Edit Reply at the bottom of the reply. Then click the little paperclip icon in the lower-left corner of the reply box. Then click Apply Edits.
2. Include a screenshot of the Edit Smart Collection window of the offending smart collection.
3. Select one of the undesired photos matched by the smart collection. Include a screenshot of the Keywording panel (in the right column of Library) showing the keywords assigned to the photo.
Without this precise information, troublehsooting via the forum will be much more difficult.
[Use the reply button under the first post to ensure replies sort properly.]