What means "1024/4096 MB allocatable"

[Mem5]

hashcat v3.00, wordlist attack :

OpenCL Platform #1: NVIDIA Corporation
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- Device #1: GeForce GTX 980, 1024/4096 MB allocatable, 16MCU

Why only 1014 ?
Does it affect hashcat performance ? copy wordlist/hashlist to GPU memory for eg. ?

[atom]

OpenCL has a bit a strange policy on memory allocation:

Max size of memory object allocation in bytes. The minimum value is max (1/4th of CL_DEVICE_GLOBAL_MEM_SIZE, 128*1024*1024)

There's no impact on performance, except for scrypt, but I've worked around the problem on there