Vintage Magnetos

Hi folks. I change my oil on my Mondeo mrk 3 2.0l saloon. Got home from work this afternoon, put the car on ramps drain oil form sump and took remove the oil filter..While that was draining i went in and had a sandwich and a cup of tea. went back out to the sump plug back in,tighten with spanner, place new filter and seal back on. lowered the car back on floor. then i place 4 litres os 5W30 semi sym oil. while that was draining to the sump done some cleaning under the bonnet. start the engine so that the filter is full of oil run the engine for about 10 minutes. turn engine off finished of cleaning. check the dip stick not a drop of oil on the stick. put the dip stick back in check oil agian nothing. I started the engine run about couple of mintues, check oil agian i had a reading. so i topped up for the reqiured amount. this never happen to me before. why is this. I always had no problems in the past. looking forward for your replys.
Thank you.