Phosphates and nitrates in tap water will fluctuate, considerably, throughout the year. All kinds of things effect tap water, including agricultural activity, snow melt, rain run off, water treatment plants treating water on a regular basis....etc. Even if you're on a well, tap water parameters will fluctuate a lot throughout the year.
There's also other elements that you need to consider, if using well water, or even city tap water. Iron is the first that comes to mind, but there's a lot of other trace elements that might or might not be good for your reef.