Varied TDS Readings

[Jaws]

I've got a Aqua Safe RO unit that I put some new filters and RO membrane in a couple months ago. I've been testing it out but the TDS readings seem to be all over the place. One batch of water will be 2 and then the next batch will be 40. I don't understand the high readings since normal tap water is only 19 or 20 so how it runs through filters and comes out with a higher TDS reading I don't know. I've got it hooked up to a sink in the basement that is right beside the washer and dryer. Is it possible that other sources like having the washing machine on at the same time is contaminating the water? This might be a stupid question but can the TDS reading climb if the filtered water sits for a period of time? Thanks for your help.