This article is about personality assessments in general, you can find information about the reliability of Alva's personality test here.
Generally speaking, the measurement of any psychological trait has some degree of uncertainty. This is also true for other measurements, for example when standing on a scale to measure your weight. I'm sure you've experienced this - two different scales might show slightly different results, depending on the quality and mechanics of the scales. However, the uncertainty is often greater for psychological measurements than physical measurements.
One cause of uncertainty in psychological measurements is, as with weighing scales, the quality of the assessment. A common way to quantify this is by estimating the reliability and standard error of measurement (SEM) of the test, which you can read more about here.
Another cause of uncertainty is simply that psychological traits are more elusive and harder to observe than physical characteristics. One cannot observe the level of Extraversion directly, for example, since this is a hidden, internal quality. Instead we rely on the self-reported answers to questions about individuals' typical behavior.
Most personality tests are designed to have a reliability above 0.8. Lower reliability indicates that there are either too few questions in the test, or that the questions don't measure the same trait. In most tests on the market, a tradeoff is made between test length and reliability. To keep the number of questions down, a lower reliability is accepted. This is not the case for IRT-based tests, however, where you can get both high reliability and a low number of questions by adaptively selecting the most optimal questions for each individual.
A high-quality test will provide results with SEM between 0.3 and 0.5, which translates to a reliability of 0.91 and 0.75 respectively. On the standard-ten scale, which is used in Alva's personality test and also by some competitors, this translates to an uncertainty of between 0.6 and 1 points. This means that you can expect your 'true' personality score to be within 1 point from the score you got when taking the test.