Like all hobbyist test kits, the pH test is only so accurate. This is primarily because of the difficulty of matching the solution color to the color chart color. But, it is also because of other combined errors. If your larger tank registers 7.6 on one test kit and 7.4 on the other test kit, it is likely that the true pH is between 7.3 and 7.7. There is no reason to care what the true pH is to any more accuracy than that. The smaller tank is more likely to be between about 6.5 and 6.9 than it is to be close to 7.4. (All test equipment is most accurate in the middle of its range, not at the ends of its range.) And, there is really no good reason to care what the pH is to any more accuracy than 6.5-6.9.
If you really feel a need to know the pH more accurately you will need to get a pH measuring probe, and at least two known, stable pH standard solutions, for calibration purposes. Measuring pH with a probe is only as accurate as your calibration of the probe.
If you were working in a chemical lab, where you really needed to know pH very accurately you would have a very expensive pH probe, and you would probably have someone who was designated as the calibration expert, who would calibrate the probe at least every week.