Greta is right. Now consider the word stop. If you say the word several times, you will probably notice that sometimes the final /p/ contains more aspiration and sometimes, less. (In fact, if you end the word with your lips together and do not release the /p/, it contains no aspiration at all.)
Since you are not pronouncing stop as part of a larger chunk of language that varies from utterance to utterance (for example, John told Mary to stop the car versus Stop and go versus When you come to the sign, stop), the phonetic environment of the /p/ remains constant--it is at the end of the word and preceded by /a/.
In other words, we cannot predict when a particular allophone with more or less aspiration is likely to occur, so the allophones of /p/ must be in free variation."
(Thomas Murray, The Structure of English. Allyn and Bacon, 1995)
# "[E]very speech sound we utter is an allophone of some phoneme and can be grouped together with other phonetically similar sounds."
(William O'Grady, et al., Contemporary Linguistics. Bedford, 2001)