I’ve worked in retail for quite a few years. Sometimes when someone makes a purchase they reject the pennies and ask the store to “pass them on” to another customer who might be short a penny or two. Say the change is $1.37; such a person will accept the $1.35, and then offer the two cents, the two pennies, in such a manner.
Sometimes when a customer is due change that is close to a round number, the cashier will give the customer a rounded up version of the change instead of the exact change. Say the change due to the customer is $5.49. Sometimes the cashier will give the customer $5.50 instead. The cashier might do this for a number of reasons. Perhaps the cashier has been given pennies by previous customers as outlined above. Perhaps the cashier is short of pennies. Perhaps the cashier is in a rush and can save a few seconds with the simpler change. Perhaps the customer is a friend. These are examples of counting, but the final results of the counting are approximations. Such approximations are common in counting. In everyday encounters we often offer approximations instead of exact numbers. For example, I might say that I saw someone “about two weeks ago”; and people understand that this is an approximation, probably because I can’t remember the exact date and time.
In poetry the same kind of principle applies. One starts with a count for a poetic form, but the specific circumstances of the poem may modify the count. If the count is accentual, and the poet is writing in iambics, a substitution may generate an extra syllable and this is considered acceptable. On the other hand, a poet might shorten an accentual line by a syllable or two for emotional effect, or to pick up the pace of the poem.
In strictly syllabic forms, the force of ordinary speech might push the line beyond the normative boundaries a syllable or two. Or, as in accentual verse, a line might be shortened to pick up speed or heighten emotional effect. As in counting change, the poet starts out with a specific result in mind. As in counting change, specific circumstances can modify that count.