I think that delta v includes an extra allowance for the extremely thick atmosphere as compared to Earth's. The delta-v for Earth is given as 9 km/s, which is actually greater than orbital velocity at 250 km altitude; the difference is the extra delta-v you have to allow for atmospheric drag. Venus' atmosphere would give a lot more drag.
On the other hand, the atmosphere of Venus could also been seen as a help since you could "just" float or fly to a great altitude instead of departing from the ground.
The ground is pretty much impossible to reach anyway, and not as interesting as the atmosphere about 50 km up, where temperature and pressure are about the same as on Earth and where breathable air is buoyant.
> the atmosphere of Venus could also been seen as a help since you could "just" float or fly to a great altitude instead of departing from the ground
Yes, but I don't think the numbers given in the chart consider such possibilities. I think they're for a standard rocket blasting off from the surface and having to account for atmospheric drag when determining how much delta-v is required to reach orbit.
> maybe that the “low orbit” distance given for Venus is 400km vs 250km for Earth
No, that won't make much of a difference: the gain is basically just the gain in potential energy for the extra 150 km, which is equivalent to only a few tens of meters per second delta-v.