Zabbix agent does not consider SReclaimable kernel option when calculate free memory. The option is like cached memory and should be considered as free memory:

MemAvailable: An estimate of how much memory is available for starting new applications, without swapping. Calculated from MemFree, SReclaimable, the size of the file LRU lists, and the low watermarks in each zone. The estimate takes into account that the system needs some page cache to function well, and that not all reclaimable slab will be reclaimable, due to items being in use. The impact of those factors will vary from system to system.

https://www.kernel.org/doc/Documentation/filesystems/proc.txt