Is there any reason for the cape to have caster level 9 and not 7?
A caster level 7 cape of the mountebank has a range of 680 ft., but a level 9 cape of the mountebank has a range of 760 ft. This probably isn't a big deal.
What is a big deal is that a cape of the mountebank
allows the wearer to use the magic of the dimension door spell once per day.
And the spell dimension door says
You may also bring one additional willing Medium or smaller creature (carrying gear or objects up to its maximum load) or its equivalent per three caster levels.
Thus a caster level 7 cape of the mountebank would only allow the wearer to bring with him two additional Medium or smaller creatures (or the equivalent) (and require "wasting" the extra caster level because caster level 7's the minimum to cast the spell hence create an item employing the spell), but a caster level 9 cape of the mountebank allows the wearer to bring with him three additional Medium or smaller creatures (or the equivalent) ("wasting" no caster levels).
Note that the GM may disallow the wearer from bringing additional creatures at all (q.v. boots of teleportation), so ask the GM if this extra capacity is a thing in the campaign or if additional distance is the only active reward for the higher caster level.
Finally, a higher caster level cape of the mountebank is harder to affect by the spell dispel magic et al.:
If the object that you target is a magic item, you make a dispel check against the item's caster level (DC = 11 + the item's caster level). If you succeed, all the item's magical properties are suppressed for 1d4 rounds, after which the item recovers its magical properties. A suppressed item becomes nonmagical for the duration of the effect.
Without increased range or creature capacity, this is likely the only reason to increase a cape of the mountebank's caster level.
Hey I Can Chan has most of the potential reasons, but there is one more. Caster level factors into the craft DC for the item. Non-standard caster levels may be placed on magic items by the game designers to increase their crafting DC so the item can only be reliably created at an appropriate level for their effect.
In this case, the DC is 12 with a caster level of 7 and 14 with a caster level of 9. However, if the creator lacks the Dimension Door spell, the DC increases to 19, which requires a fair amount of investment to reliably meet. This may have been the developer's thought process.