The quick answer is yes, we indeed see real changes in the structure of the jet.
Longer answer; There are several factors that define how the jet of 3C 273 looks like: frequency of observation, telescope/array used, and real changes in structure.
If we observe with interferometry technique from Earth, the higher the frequency - the better the resolution and the worse is sensitivity. In simple words, at higher frequency one can see finer but only the brightest details.
The images you’d shown are indeed taken with some VLBI array: a number of telescopes observing simultaneously at the same frequency. The more telescopes one has at once - the bettter image is. With less telescopes or problems even at a single telescope of the array, the image is more noisy
Finally, the jet itself changes. In case of 3C 273, the changes are visible with monthly observations.
There are two excellent programs to monitor jets and see real structural changes:
The matter in these jets indeed moves with immense speed, exceeding 0.99 of the speed of light. But because the jet in 3C 273 is pointing almost towards us, there is another funny effect that increases apparent speed: https://en.wikipedia.org/wiki/Superluminal_motion
It can be imagined that emitting plasma and its emission are moving towards us almost at the same speed and on almost parallel trajectories. In projection on the sky plane, the apparent speed of any feature in the jet would be larger than the speed of light. It does not imply that the matter moves faster than the speed of light. It is just an apparent effect.
The measurements of the apparent speed are made on the basis of positions of bright spots/knots moving along the jet. The most probable nature of these bright features is shock waves traveling downstream the jet.