Manufacturer provided DPI vs ES Task Manager displayed DPI

Question

I am a web developer and I am looking for an android phone below Rs. 10,000 for my development purpose which should have 360x640 viewport size.

I don't find any phone manufacturers specifying viewport size in specifications. So, I decided to calculate viewport size with manufacturer provided information like screen resolution and dpi.

I have a Samsung J7 Max android phone. I used this phone to find a method to calculate viewport size.

Below is the example phone specification:

Model : Samsung J7 Max

Screen Resolution : 1080x1920

DPI or PPI : 386

Refer this site for full specification: https://www.gsmarena.com/samsung_galaxy_j7_max-8684.php

I Googled and found below formula to calculate the viewport size.

dp = px / (dpi / 160)

This is how my calculation was,

Width:
dp = 1080 / (386 / 160)
dp = 448

Height:
dp = 1920 / (386 / 160)
dp = 796

So, the viewport size would be 448x796.

Now I opened http://whatismyscreenresolution.net/ in my mobile browser to check the viewport size. And it returned 412x732.

??????

Calculated viewport size : 448x796

Browser viewport size : 412x732

The calculated viewport size is different from the viewport size displayed in the browser.

So I went back to my calculation and checked every number. But I couldn't spot out what was wrong.

I installed ES Task manager app in my phone to verify dpi. Surprisingly it showed 420dpi, where 386dpi was in phone specification.

Now I performed the same calculation with dpi as 420(ES Task manager displayed dpi value).

Width:
dp = 1080 / (420 / 160)
dp = 412

Height:
dp = 1920 / (420 / 160)
dp = 732

Now calculated viewport size matched the browser(412x732).

Now my question is,

1. What is Manufacturer provided dpi value(386) and ES Task manager displayed dpi value(420)?
2. How to come up with a number 420 from the manufacturer provided information?! so that I can match the browser viewport size?!

Thanks a lot!