There are a lot of variables that could affect pH. I'm just beginning testing on a product from flairform called greendream which is a one part slurry. Apparently it has a very high pH buffering capacity. I found the following on their website which may help.

"Ever wondered why pH fluctuates (i.e. typically upwards) after it is lowered? This behaviour is actually a consequence of adjusting the pH. Lowering pH via adding acid, removes bicarbonate and produces carbon dioxide. The presence of this free (i.e. uncombined) carbon dioxide (CO2) tends to lower the pH because it reacts (only weakly) with water to form carbonic acid. However, CO2 concentrations above about 0.5 mg/L in water are unstable when such waters are exposed to the atmosphere (at sea level pressures). Under that condition CO2 in excess of 0.5 mg/L will slowly escape from the water into the atmosphere. Consequently this loss of acidity causes a corresponding rise in pH.
This subsequent rise in pH is particularly noticeable with ground waters (i.e. bore water) which typically have CO2 contents around 50 - 200 mg/L (due to biological activity within the aquifer). When these waters are pumped to the surface, the pH rises with time because the excess (acidic) CO2 gradually escapes (Fig 1.16). The pH will then rise to a stable value solely dependent on the water's bicarbonate content.
Example: A bore water with 100 mg/L bicarbonate and 100 mg/L of free CO2 will have an initial pH of 6.3. Its pH will gradually rise to 8.2 after it has been exposed to the atmosphere for sufficient time to allow the CO2 content to drop to around 0.5 mg/L.
The same phenomenon (although to a much lesser extent due to lower CO2 contents) can occur with scheme (tap) water. Thus the conclusion â?? because the pH of waters is only stable after aeration, it is only the "after aeration" pH value that has any interpretative significance. To determine that value, aerate the water by tumbling a sample of it from one container to another, 30-40 times prior to measuring its pH.

Conclusion: Interpret pH values with caution because a water with a lower pH than another may produce the higher pH after both are aerated!!"

The pH value of raw waters is meaningless!

As far as the Aerogarden is concerned I'm guessing pH is 'controlled' using the nutrient tabs which must have some type of buffering capability. User pictures of plant health have left me unimpressed. I'm guessing it's a bit like a McDonalds burger.. they look great on the ad's but the real thing is a complete disappointment.