Employment rates and unemployment rates are not the inverse of one another.
The unemployment rate measures the percentage of those wanting to work that have work. "Full employment" means that all those who want work get it, but there is always some level of friction involved in entering the workforce or moving from job to job — someone completes the education/training in June but doesn't find, and start, in a new job until September; a worker is made redundant and some time passes before they find and start another job — and this is generally reckoned to be about 5%, so if the unemployment rate is 5% or less, labour economists etc will generally consider this to be "full employment".
The employment rate is the percentage of the population of working age who have work. But a non-trivial proportion of the population of working age don't want work, for a variety of reasons — they are studying, they have a disablity, they are homemakers with young children who do not want to put them into daycare, they are sufficiently wealthy that they don't have to work and they choose not to, they are carers for sick or infirm relatives, Consequently, no matter how thriving the labour market and how close to full employment the economy is, the employment rate will always be well below 100%.