probably the size of the pots you're using. if you start them in 5gal buckets they are muc hbigger after a week than if started in rockwool cubes or smalll pots
for the test kits you need something that will let you know if in the ranges of 5.5-6.5 or a little broader, but pools usually have 6.4 the lowest so they aren't very useful. Order some cheap strips or a more expensive digital meter, or just bank on the fact that most tap water is 6.5. Digital meter is best though, I need one too.