The Press 'n Seal is to hold moisture, not air, in (moisture is bigger than air). My guess is that you have achieved a higher humidity than you think. Many hygrometers are inaccurate when they reach levels of saturation (like 85+%, for example). I pretty much saturate my substrate under the light diffuser (no Press 'n Seal, just locking lid tubs), but remember that eggs will weigh it down slightly and you don't want them contacting the water directly (therefore, no water floating on top of the substrate).
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