You're most likely spot on about the cultural reasons for the behaviour, I just wanted to point out that it doesn't come across to me as nice. It comes across as passive and obstructive.
And I'm English, we're pretty good at passive...
Actually, I just want to point out if you've had any training on how to run a training session, you ought to know that asking "do you understand?" is pointless in every culture. From an asian culture it can even come across as threatening and dominating. Its like asking "do you want to keep your job?". Who is going to stand up and say "actually, no I don't" to that? Work in a western company is often the very first time in peoples lives where voluntarily admitting you don't know something doesn't carry significant penalties.
I've never phrased it as confrontationally as you're assuming, it's always been a genuine offer of help.
Never has been pointless when dealing with European, American or Australian resident engineers (regardless of racial or cultural background), it's simply opening the floor to new questions. So now's who's guilty of sweeping assumptions?
Regardless, the original point still stands - this is not niceness, it's cultural conditioning to be non-confrontational to the point of dishonesty, and it's highly counterproductive.
I guess my point is that unless you're well versed in the culture, it's easy to trip over these things.
I have no doubt it's a cultural misunderstanding rather than an attempt at deceit, but then... how the hell do you communicate technical knowledge to people who won't tell you what it is they need to know from you?
Well, I have been in that same boat. My answer? Don't hire them in the first place, and try and avoid projects if that decision is made for you.