You're still lucky to get within 2cm. As others have said, there are multiple factors involved. GPS RTK elevations cannot in ANY case be relied upon to be within about .10', or about 3cm. This can vary by quite a bit more, though, if we're getting solar flares (which are happening to a large degree right now, and will continue to happen for a few more years).
And ALL elevations from GPS depend on your geoid model, at least if you are trying to cover any significant area. The 2009 geoid is the best-yet, but it can still vary quite wildly, depending on where you're at. It gets worse in areas of large vertical grades, such as areas near where I'm at, close to the continental divide (in Colorado). It can also get quite bad if there are large deposits of various ores in your local area. This might not be so significant for RTK construction stakeout on small sites, but it can make a world of difference when setting control or working over large areas.
When it comes to construction stakeout, there are times when you can use GPS RTK elevations, and times you cannot. If you are in an area where there's lots of slope, then .10' of error in a curb stake might be unnoticeable, because construction crews can't achieve the level of accuracy we can achieve with good survey equipment, but also because there's enough slope to make sure everything drains. In flatter areas, you can easily create bird-baths using GPS elevations. Far better to set control around the site (and you can use long-term GPS observations for that, vs. RTK), then use a good robotic total station (with max 400' shots) to set the rest. If you don't have a good robot, or you want to go further in your horizontal layout, then you maybe should use a level. We don't use the levels as much as we used to, because the robots we have are so good these days, but there are times when you should. We still have a digital level that has served us very well in certain circumstances.
As always with Surveying, it requires knowledge of what you're doing, so you can make correct decisions in each case. That's what we do, as professionals. Any licensed Surveyor should know WHEN we can make one decision over the other, and what the ramifications are. Otherwise, we could just send the average person out with a hand-held GPS device. (And then see how many nightmares result from it.)
