Why can verbs with imperfective morphology have a perfective meaning?

The examples you chose are not particularly fortunate but the problem you're alluding to is one commonly encountered when it comes to aspect or tense. For instance, present tense is often used to refer to future events "We're leaving at 5 tomorrow." And even here you could argue that the aspect is misaligned too. And there are many instances in Slavic languages where verbs with imperfective morphology have broadly perfective meanings (and vice versa).

The problem you have is that you chose a verb of motion which has the following properties in most Slavic languages:

1. has separate forms for imperfective and repetitive
2. does not have a clear perfective counterpart
3. has irregular non-analytic future tense

This has led many people to perceive what is essentially an imperfective verb as having perfective meanings or being perfective in the first place. I've encountered this intuition among native speakers as well as some experts.

This is not helped by the fact that the literature on aspect is very sketchy and ignores much of the data available. The fundamental problem is a fixation with what people often refer to as 'perfective' or 'imperfective' meanings. They're just too general to make sense in every context.

I find a functional constructionist approach much better to help make sense of the extremely rich and varied area of aspect usage. This combines the morphology and lexicon of aspect and tense with the semantics situated within the TAM (Tense Aspect Modality) space. You don't then ask what is the aspectual meaning of this verb but rather where in the TAM space is a particular Tense-Aspect construction located. For example, you could say Repetivitive-Past is used to talk about past experience not continuing in the present, Imperfective-Present is used to talk about historical narratives, Perfective-Past is used to talk about negatively or emotionally evaluated past repeated experiences. Each of the tense-aspect combinations have multiple such meanings (similarly to English) - I gave the non-prototypical ones - some of which seem directly in contradiction to what you'd predict from the aspectual meaning. Which is why it makes more immediate semantic sense to look at them as separate but related constructions than if you just look at the abstract meaning of 'aspect' and try to go from there.

I did a more detailed analysis of Tense-Aspect-Modality in Czech here. The same principle would work for Russian but the individual constructions would be different - which makes for a more principled cross-linguistic comparison.