The tech for detecting AI generated text

How can a computer program like GPTZero can tell if a text was AI generated or now. These class of softwares are guided by 2 ratios:

• perplexity: a function of "how surprising" is this language given the training data. Given the phrase "I want a cup of _". Continuing with a word like "tea" means low perplexity. "a cup of spiders" however is more surprising, and thus human. 
• burstiness: evaluates the variability in sentence length and structure throughout a text. AI tends to be more omogenous aka boring and rapid changes in sentences length, the use of adjectives here but not there are more human.