A number of things can affect the reading - firstly, heavy rain can affect the reading, usually taking around a week to show up on the tds meter, secondly the temperature of the water,warm water will absorb minerals more readily, thirdly, mains water work can have a bearing both increasing and decreasing tds (changing mains source etc). There are a 1001 reasons why the tds levels can rise and fall but these are the main reasons. Also the time of day can affect tds levels as in the evening when less people are using their water supply the pressure tends to rise, as the pressure is greater the tendency for water to absorb more minerals from pipework decreases as the water works its way towards your home. Thats why many people find that their RO is more productive at night as pressure is greater and tds is somtimes slightly lower, with saying this water temp can drop at night having a slightly negative effect on production but overall the RO will perform better in the evening for the majority of people.
Make sense??