Few possibilities to consider:

1. NS-2 Version: Different versions of NS-2 might have variations in their implementation or default settings, which could lead to differences in simulation results. It's possible that the versions of NS-2 used in Ubuntu 14.04 and Ubuntu 16.04 have different default configurations or behavior for certain components of the network simulation.

2. System Libraries and Dependencies: Ubuntu 14.04 and Ubuntu 16.04 may have different versions of system libraries and dependencies required by NS-2. These library versions can influence the behavior of the code and the simulations. If NS-2 relies on specific library versions, inconsistencies between the versions installed in the two Ubuntu releases could result in different outcomes.

3. Compiler and Toolchain Differences: The different Ubuntu versions may include different versions of the GNU Compiler Collection (GCC) or other build tools. These tools can affect the compilation and execution of the NS-2 code. Variances in compiler optimizations, code generation, or default compiler flags can lead to variations in the behavior of the simulation.

4. Kernel Differences: Ubuntu 14.04 and 16.04 likely have different Linux kernel versions, which can impact the low-level system behavior. NS-2 interacts with the operating system and the network stack, so variances in kernel behavior might influence the simulation results.

5. Environmental Factors: Other factors such as system configurations, installed packages, or runtime environment settings could differ between the two Ubuntu versions. These differences might indirectly affect the behavior of the NS-2 simulation, resulting in different outcomes.

Please compare the NS-2 versions, system configurations, library versions, compiler toolchain, and kernel versions between the two Ubuntu installations. It should help you understand why the same code produces different results.

Good luck!!