depends on how you are checking your rankings and how did you checked the searches per month for the keywords, may be rankings you are checking are personalized results and may be there are less exact matched searches for the keyword and adwords keyword tool is not that updated so may be the searches for the same keywords have reduced in the past few months and is not updated with adwords keyword tool. hope that helps...
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You should first check your website home page content for duplicate.If it is unique then check your meta tags and keyword for your region.
If you receive 500 visitors in a day, so your monthly visitors are 15000, i think it is good traffic.
The adwords keyword tool and any traffic estimator won't be perfect. Everything is constantly changing. Also keep in mind that you should be looking at EXACT match traffic.
Thats searches count as broad searches in adword. you should check as a exact match searches about the keywords.