UHF RFID reader ability to recognize tags in less than 1 m reading distance
Because an RF reader can read tags at 5m is doesn't mean it can do so for a passive tag. I'm saying this because the reader specification may be referring to an active tag i.e. one that contains a battery and is able to wake up when it detects the smallest "proximity field" from the reader.

So, once you've got that firmed up, adjusting the range to 1m (and this is usually done by lowering the "proximity field"), may give variable results - some may work outside 1m and some may not work until well-inside the 1m range.

I'm assuming your question is "What factors should be considered when reading a tag at a certain distance".

A tag relies on a wake-up field (or proximity field) to activate it and, the detected level of this field will vary from tag to tag - it's worse for passive tags because they are energy harvesting the field in order to create enough internal supply voltage for them to transmit a response and therefore there are more factors that can vary when energy harvesting.