In the reaction with 1-pentene, there is a different degree of alkyl substitution on the two ends of the C=C double bond. As such, HBr has some basis for "making a decision" -- according to Markovnikov's rule -- in terms of which regioisomer it should produce. Hydrogen ends up on the terminal carbon atom, which had one alkyl substituents to begin with, and bromine ends up on the interior carbon atom, which had one alkyl substituent at the outset.
In the reaction with 2-pentene, the degree of alkyl substitution is the same for either carbon atom in the C=C double bond: each has one alkyl substituent. As such, the bromine atom is just as likely to end up on carbon #2 as it is to end up on carbon #3, and a mixture of regioisomers results.
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