Difference between AWS amazonei_tensorflow and tensorflow conda environments
The amazonei environments are for use with the Amazon Elastic Inference accelerators. You can see this mentioned in the DLAMI README file:

for TensorFlow(+Keras2) with Python3 (CUDA 9.0 and Intel MKL-DNN) ______________
____ source activate tensorflow_p36

for Tensorflow(+Amazon Elastic Inference) with Python3 _________________________
____ source activate amazonei_tensorflow_p36