Your OG will be as the recipe states when brewing with extract if you used all of the fermentables and the volume is correct in the fermentor. It is difficult to get all the extract mixed well to have a SG sample that is not either high or low in sugars. The yeast will take care of the mixing. I don't check OG anymore when brewing all extract.
You may have exactly 5 gallons in the bucket or you may not. The factory markings on buckets, kettles, etc. are often incorrect. I marked all of my carboys at the 5 gallon level by adding an exact measure of water to each one. Might be worthwhile to add gallon marks to your bucket before the next brew.
edit: Maintaining a stable fermentation temperature is the thing to watch now.