First, it is worth noting that non-exercise tests are especially inaccurate at determining both VO₂max and changes thereto. One study examining a host of different methods, including that described by Jackson (1990), concluded:
"changes to [estimated cardiorespiratory fitness] (eCRF), as
determined using non-exercise prediction equations, were significantly
associated with changes in directly measured [cardiorespiratory
fitness] (CRF). However, changes in eCRF values from most of the
prediction equations were significantly different from the changes in
directly measured CRF. Furthermore, all of the prediction equations
had a low degree of accuracy when identifying even the directional
change of CRF. These findings highlight the errors associated with
non-exercise prediction equations, especially related to monitoring
longitudinal changes, and suggest limited prognostic utility of eCRF
within a clinical setting."
Exercise VO₂max tests are therefore more accurate, but without knowledge of the precise method employed by your stationary bicycle, it is impossible to assess its accuracy and/or limitations.
In sporting circles, VO₂max is typically tested directly with a multi-stage maximal test, but this requires specialised equipment and tester expertise. In the field, this can be replaced with a multi-stage submaximal test, such as the YMCA Submaximal Cycle Ergometer Test, which estimates VO₂max through extrapolation from submaximal efforts. Intervals are typically three to five minutes, since a maximal three-minute effort is around 95% aerobic and a four-minute effort around 99% aerobic.
I hope that helps, even though I can not give you more information about your particular stationary bicycle.