Physics is very nice except when it isn’t.

Generally, future location/direction/trajectory is very convenient to calculate when there aren’t iffy factors involved, such as drag. The deep of space where the roadster is located has practically no drag so we can accurately predict its location, low earth orbit however still has quite a bit of Earth’s atmosphere there so drag becomes far bigger of a factor.

Drag is relatively difficult to calculate because it depends on, for example, the way the object is oriented relative to its direction of travel, the density of the air which isn’t necessarily always uniform, winds which can’t always be tracked, and basically any other properties the air has. As a result, it is hard to account for drag, particularly because over time these fluctuations, which are all the more common in the upper atmosphere.

During small periods of time, these fluctuations aren’t that much and we can deal with them pretty easily, such as when landing falcon boosters, we have computers nearly instantly figure out that they are off from their necessary trajectory and adjust accordingly. The falcon rockets didn’t just thrust one way according to earlier accomplished calculations, then thrust as the end to slow it down, the rocket was constantly compensating for slight differences in trajectory caused by drag and what not to land accurately enough.