Why does CPUUtilization is more than EngineCPUUtilization for ElastiCache and Even though memory is available, SwapUages is above zero?

Question

I am trying to understand the use case in which EngineCPUUtilization Matric is more relevant than CPUUtilization for AWS ElastiCache. Since Redis is single threaded, it is expected that EngineCPUUtilization would peak apriori the CPUUtilization, but in my monitoring I can see both these matrices having similar values. Can any one help me understand the exact use of these two matrices. Also even though, the memory is available, SwapUsages matric has non zero value. why is this? and does SwapUsages matric is used even if physical memory is available which seems contrary to usual swap definition.