Keras Model Predict is not predicting all images flowing from directory?
I presume that your generator is at fault here :

predIdxs = model1.predict_generator(testGen,
    steps=(totalTest // 32))


You do an integer division on the size of your test set, but the result is not an integer and thus truncated (floored). Later on you (presumably) use original size for one column, and want to assign the data from your predict_generator (which is shorter) to another column of that same dataframe, creating a mismatch.