Short Term Trading Weekend Lounge: 6 -8th Apr, page-34

Two ways to look at that @pmd3nka - the cross is valid because the macd crossed the signal line, the macd calculation is the difference between two moving averages (26,12) with a signal line generated by a 9 period ema of the difference between those averages.

So we can have (and always do) a period where the averages are not equal but they have crossed the signal line, either declining or advancing. The zero line is drawn where the two (26,12) ema's are equal and this will usually be different from the signal line period due to this (9 period signal line) delay. If the price does not change for an extended time then these levels will be the same.

It's up to the trader to be aggressive and trade the crossover or wait for the averages to be equal (the zero line) - Most scans will always give the macd x-over as posted by @Nodferatu.