No Click Sound on First Press of Passcode

I have recently purchased an Ipad Air 16gb wifi. All well except for an odd problem. When waking Ipad, when you press the first number of the passcode you do not hear a click sound, you do hear a sound for the second to fourth numbers pressed. I have spoken to Apple support, I have had unit checked at an Apple retailer, all agreed it was odd behaviour and they had not heard of it before. After two or three factory resets and reloading of firmware, all to no avail, they replaced my Ipad, the new unit also displays this behaviour. If you put it into standby and take it out of standby quickly it works, all four click sounds can be heard, if it is in standby for more than 10 secs before waking only three click sounds can be heard on the four digit passcode.

Obviously it's not a big issue, could have hidden something though more serious, I believe this is a firmware issue, not many reports of this problem though on the forums etc. So my question is, are there Ipad Airs out there WITHOUT this problem ? or do they all display this behaviour and nobody is bothered about reporting it ? My Ipad 2 with 7.1.2 has never misbehaved like this.