Why does the sqlserver process show less memory usage than it actually does?

Question

PHP

Good afternoon.
I have Windows 7 x64 with 16GB of RAM. It has the only SQL server 2012 that has 10GB of memory allocated in the settings. For a couple of months, he ate everything. that required work with a DB on it and everything worked quickly. But recently, the task manager began to show that the process eats up only 300-400MB, depending on the load, and the request

select @@servername,cntr_value/1024 [memory_sqlserver, mb] from master..sysperfinfo
where counter_name = 'Total Server Memory (KB)'

shows 3877mb (this number also floats).
1. Why can this happen?
2. Why did the amount of used memory drop sharply if the load has not changed, and the database is growing every day? 800 mb per day.

Answer 1

$result = $mysqli ->$query($mysqli,$sql);

$result = $mysqli->query($mysqli,$sql);