Within reason differences in development are the default situation during periods of technological development, absent something to homogenize them like a shared national government. Somebody is going to innovate first and it takes lots of time for new ideas to diffuse.
In the Neolithic era, for example, food production technology from domesticated plants and animals took about 1000 years to diffuse from expanding first wave Neolithic farmers to neighboring hunter-gather populations with whom they were in contact, who were also often reduced in numbers but not obliterated entirely.
In the Bronze Age, the Hittites kept their iron working skills secret from everyone else in a thousand mile radius for many centuries and used that to give them a military edge and expand their empire.
In the early modern period, China experienced bureaucratic stagnation and went from being a technological leader to halting virtually all technological advances for centuries.
There is a huge diversity in levels of development today. While some of the more extreme examples involve uncontacted people (Andaman Islanders, isolated Papuan tribes, Amazonian jungle tribes), even with regular contact there are huge disparities in development. For example, cross the border from California or Texas to Mexico, take a boat from Florida to Haiti, compare Afghanistan to pretty much any other place in Eurasia, compare Egypt or Algeria or South Africa to Cameroon or Zaire.
Huge gaps aren't unprecedented even within a single country, for example, at the moment there are huge gaps between the U.S. Appalachians or Mississippi and Silicon Valley or New York City; between rural inland China and urban coastal China; and between different states in India.
With contact between more and less developed areas, however, you do get weirdness.
For example, in Somalia you have cell phone based banking and widely worn manufactured rubber soled shoes existing side by side with animal powered farming, thatch huts and outhouses (or just shitting outdoors in fields and alleys). In Nigeria, you have both Internet access and widespread extralegal witch trials. In Nepal, traditional "witch doctors" practice in the same villages and medical doctors employing allopathic medical techniques.
In general, some technologies and economic arrangements and life styles are more portable than others.
One doesn't need complete geographic barriers to establish cultural and technological isolation. For example, the thin shallow strip of ocean between China and Japan, together with regular typhoons, proved a significant barrier to invading armies that allowed Japan to avoid Chinese hegemony and conquest at their hands for many, many centuries even as trade between the two countries and bride exchange continued steadily during that time period.
More advanced countries may be ideologically opposed, or just not bothered given to the cost involved, to conquer other countries. And, some technologies may be better suited to some conditions than others. Kansas style farming doesn't adapt very naturally to life in the Amazon or the Congo. It is harder to build highway systems or rail systems in mountainous areas than on flat plains.
Ideology and history and local conditions also influence the path that development takes. Islam retarded portrait painting and mortgage financing, but in the early Islamic empire you could use the equivalent of checks drawn in Morocco in Indonesia, abstract art was advanced, and algebra flourished.
The United States has an awesome interstate highway system, good commercial aircraft service and world class universities, but lousy passenger trains and intracity bus systems, and relatively mediocre K-12 education and public health institutions.
There is no reason that this couldn't be true in your world.
For example, in the steampunk genre, it is almost cliche for one country to advance mechanical sciences while a rival develops biotechnologies instead. Another common trope is to have one place use magic while another relies more on science.