Who said it's random? ... depending on the current trend direction it would either be below or above the market.
yeah it's either boy or a girl random, same way prices go up or down random
If prices depend on the current trend direction - they are not random - and this is different from boy/girl births which are random.
Your question has been perfectly answered - price lags because of low volume. Low volume means bigger volatility, so naturally other exchanges would have price jumping around mtgox average, depending on the current trend direction it would either be below or above the market.
Additional data - TradeHill does not visibly lag after mtgox in the way you describe. It fluctuates around the mtgox price but it has not been consistently below mtgox for a month despite the current trend direction. It does not seem to be simply about volatility.